Rational Drug Prescribing

Rational drug prescribing means giving each patient the right medicine for their condition, at a dose suited to them, for long enough to work, and as cheaply as possible for the patient and society (the WHO idea of rational use). The environment in which drugs act is complex, with many potential interaction partners, and improper selection can lead to severe complications. Irrational drug use is associated with adverse consequences including drug resistance and avoidable adverse dr

Poly-pharmacy is defined more concisely as the practice of prescribing four or more medications to the same person. It acts as a major measure of rational drug use because it increases the risk of adverse drug reactions, severe drug-drug interactions, non-adherence to treatment, and prolonged hospitalization. Chronic medical conditions such as cardiovascular disease, diabetes, and psychiatric disorders frequently require such multiple drug use for prolonged periods, yet this must be carefully monitored to avoid harm. Factors associated with irrational drug use include direct demand from patients, patients' deep-seated beliefs about injection drugs, and the persuasive influence of pharmaceutical sale representatives. Furthermore, the adverse effects of irrational drug use are enormous, encompassing the wastage of limited healthcare resources, the transmission of blood-borne infections through the inappropriate use of injections, an increase in resistance to anti-malarial and anti-microbial drugs, and a rise in overall drug-related morbidity and mortality. Concerted efforts are therefore required to improve these metrics, particularly regarding the total number of drugs prescribed per encounter and the proportion of drugs given in injection form. Regular training programmes should be organized for health care professionals to promote rational habits, while governments should formulate regulations to control the activities of pharmaceutical sale representatives. ^{[2] [4]}

Rational drug prescribing is the systematic application of evidence, pharmacology and patient-specific factors to select the most appropriate medicine, dose, formulation and duration for a given clinical situation. The World Health Organization describes rational drug use as each patient getting the medicine their condition needs, at a dose that suits them, for as long as it takes to work, and at the least cost to them and to society. Irrational prescribing is the deviation from this standard, and it carries serious consequences.

The core components of rational prescribing can be broken down into several measurable indicators. These are used in clinical audits and research to assess prescribing quality:

• Mean number of drugs per encounter – the average count of medicines prescribed at a single consultation. A high value suggests polypharmacy, which increases the risk of drug interactions and adverse effects. In one Nigerian psychiatric hospital study, the mean was 3.5 drugs per encounter. ^[8]
• Percentage of drugs prescribed by generic name – generic prescribing reduces confusion during dispensing, improves drug inventory control, prevents sudden stock-outs, and makes purchase easier and more affordable for the patient. In the same study, only 58.5% of prescriptions were generic, well below the 94% reported in a Zimbabwean study. ^[8]
• Percentage of prescriptions in injection form – injections carry risks of infection transmission and are more expensive; they should be reserved for when oral therapy is impossible or inappropriate. Over a third of prescriptions in the Nigerian study were injections, compared with 25% in a Syrian study. ^[8]

The consequences of irrational drug use are substantial and well documented. They include wastage of resources, transmission of infections through inappropriate use of injections, increased resistance to anti-malarial and anti-microbial drugs, and increased drug-related morbidity and mortality. Drug resistance is particularly concerning because it erodes the effectiveness of first-line treatments and forces the use of more toxic or expensive alternatives. ^[8]

Several factors contribute to irrational prescribing. In the Nigerian study, although all doctors and pharmacists were aware of the concept of rational drug use, only a small proportion were actively receiving updates on the topic (doctors 11.1%, pharmacists 23.8%). All prescribers believed they needed more information on the concept, compared with 85.7% of pharmacists (P=0.004, 95% CI=1.031–1.32). This highlights the need for continuous professional education. ^[8]

Improving rational drug use requires a multi-pronged approach: regular training programmes for health care professionals, educational programmes for patients, and government regulations to control the activities of pharmaceutical sales representatives. National policies can be effective—one national policy on rational drug use successfully reduced antibiotic prescribing for common but questionable outpatient conditions. ^[8]

The World Health Organization provides a structured framework to help clinicians make rational therapeutic choices. The first step in this logical pathway is to define the patient's problem precisely by making a thorough diagnosis based on signs, symptoms, and investigations. Once the exact diagnosis is established, the clinician must specify the therapeutic objective. Setting clear goals ensures that both the prescriber and the patient understand whether the treatment aims to cure an infection, relieve chronic symptoms, or prevent long-term cardiovascular events.

The third step involves verifying the suitability of the chosen treatment. This requires checking standard pharmacological treatments against the patient's physiological profile, existing comorbidities, and potential drug-drug interactions. Because poly-pharmacy is a known driver of severe interactions and adverse drug reactions, evaluating whether the patient is already taking four or more medications is a critical safety checkpoint. Step four is to write a clear, legal, and unambiguous prescription, ensuring accurate dosage forms to avoid the risks associated with unnecessary injections or non-generic formulations. Step five requires providing appropriate information, instructions, and warnings to the patient. Informing patients about their medications improves adherence and prevents treatment failure. Finally, step six involves monitoring and stopping the treatment when necessary. Clinicians must evaluate whether the therapeutic objective has been achieved or if adverse effects mandate drug withdrawal. Educational programmes aimed at improving awareness among patients on the importance of rational drug use should be implemented alongside these clinical steps to guarantee optimal therapeutic outcomes. ^[2]

Rational drug prescribing requires a structured approach to selecting medicines, ensuring that pharmacological treatments are both safe and effective for individual patients. When evaluating prescribing habits, data shows that more than half of the drugs, specifically 58 percent, were prescribed using their generic name. This figure is comparable to a study conducted in Ghana which documented a rate of 61.1 percent. Prescribing generically is beneficial because it helps to reduce confusion during dispensing, improves control over drug inventory, prevents sudden out-of-stock events, and makes drug purchasing much easier and more affordable for the patient. ^[2]

However, comparative studies highlight regional variations; for instance, a study in Zimbabwe reported a much higher generic prescription rate of 94 percent. In psychiatric settings, traditional prescribing patterns often rely heavily on conventional medications due to cost and availability, yet over a third of encounters feature at least one injection drug in the prescription. ^[2]

Clinical audits also reveal operational hurdles. For example, all respondents surveyed believed that the average number of drugs prescribed per encounter in the outpatient clinic was high, and 97.9 percent believed that the use of generic names in prescriptions was low. Furthermore, all respondents agreed there is a high percentage of injection drug use at 100 percent, a low percentage of prescribed drugs from the essential drugs list at 100 percent, and that pharmacists do not spend enough time with their patients. ^[2]

When addressing complex regimens in vulnerable populations, structured interventions are vital. One care home study demonstrated a significant decrease in the drug burden index by 0.34 observed six months after implementing a pharmacist-led deprescribing intervention for residents^[1]. ^[3]

Prescribing medications safely requires understanding how prescriber experience and external pressures influence clinical practice. Junior doctors with fewer years of clinical experience accounted for a significantly higher proportion of prescribers who prescribed four or more drugs per encounter, reaching 59.7 percent compared to 40.3 percent for senior doctors and consultants. Furthermore, junior doctors represented 61.3 percent of those prescribing drugs in non-generic names, whereas senior doctors accounted for 38.7 percent. ^[2]

Clinical encounters frequently involve complex decision-making regarding the total number of medications. Data analysis showed that prescribers recommended 4 or more drugs in half of the encounters with patients, and the average number of drugs per encounter was 3.5. Similar averages of 3.5 drugs per encounter were reported in the Central African Republic, while studies in India and Ghana reported 3.2 and 3.6 respectively. Higher averages reached 8.6 drugs per encounter in Brazil, whereas a more recent study in Nepal reported 2.9 drugs per encounter. ^[2]

External factors heavily influence prescribing habits in hospital clinics. A substantial number of physicians agreed that most prescriptions were patient-driven, as healthcare professionals attributed polypharmacy to the pressure and demands of patients who request the inclusion of certain medications. Similarly, non-generic prescriptions often stem from the influence of pharmaceutical companies and representatives who mount immense pressure on prescribers to promote specific commercial brands. ^[2]

Accurate dose calculation requires proper evaluation of calculation geometries and the total number of monitor units required to reach the prescribed dose. All treatment plans are normalized with respect to the mean dose in the primary target volume. When assessing dose calculation suitability for cone-beam computed tomography systems, the impact of cone-beam computed tomography number accuracy and reproducibility is investigated for phantom data and multiple patient populations. Isocentric plans such as one field, four fields, and clinical seven field intensity-modulated radiotherapy are applied to planning computed tomography first, and beam arrangements including calculated monitor units are transferred to the cone-beam computed tomography phantom geometry.

Orthogonal dose planes are calculated in coronal and sagittal orientation with a source-plane-distance of 100 cm. For phantom images acquired with specific presets, applying preset based tables reduced the mean difference in dose from 19.9% plus or minus 2.5% and 18.4% plus or minus 3.7% down to 4.2% plus or minus 1.2% and 2.6 plus or minus 0.7%. Larger deviations are found for one field techniques than for four field techniques due to volumetric regions of interest located within inhomogeneous dose distribution areas. Intensity-modulated radiotherapy dose calculation showed the largest deviation for standard computed tomography table values reaching 8.8% plus or minus 6.3% and 6.8% plus or minus 5.6%. ^[6]

Application of phantom specific tables improves calculation accuracy significantly, yielding differences of about 10% for pelvis and thorax groups compared to differences less than 5% for head groups. Patient specific density tables show good correlation with mean dose differences of 1.20% plus or minus 0.91% for pelvis, 1.72% plus or minus 0.99% for thorax, and 1.36% plus or minus 1.96% for head patients. ^[6]

Rational drug prescribing in special populations, particularly older people living with frailty, requires a fundamental shift in clinical priorities. Unlike general adult populations, the primary therapeutic goals in frail older individuals often pivot away from reducing disease risk and prolonging life towards minimising treatment burden and preserving overall quality of life. This re-evaluation is essential because frailty and polypharmacy share a complex, bi-directional relationship. Physiological alterations, multiple concurrent pathologies, varying life expectancies, and fluctuating functional or cognitive statuses interact closely with drug therapy. These physiological networks directly alter pharmacokinetics and pharmacodynamics, thereby modifying drug toxicity and therapeutic efficacy, which can subsequently worsen frailty itself.

Clinical management is further complicated in multimorbidity, where patients frequently receive care from several distinct healthcare providers^[1]. Poor communication across specialties and cooperation barriers with medical specialists who strictly champion 'their' specific disease guidelines represent major obstacles to successful deprescribing^[1]. Furthermore, clinician and patient beliefs regarding preventive drug therapy vary widely; some acknowledge that while the benefit for an individual patient may be modest, it remains worthwhile at a population level^[1]. Providing structured information regarding potential harms and benefits successfully enhances patient knowledge and diminishes decisional conflict, although it does not consistently alter the final decision to initiate or continue treatment^[1].

Polypharmacy presents significant challenges in clinical practice, particularly for vulnerable patient populations. The process of tapering, dose reduction, stopping, or switching drugs is known as deprescribing. This structured intervention aims to manage polypharmacy and improve overall clinical outcomes. Specific tools such as STOPPFrail have been developed to assist physicians when making deprescribing decisions for individuals living with frailty. ^[3]

When examining safety, studies show that deprescribing is feasible and safe among older people with identified frailty, as it does not adversely change hospitalisation or mortality rates. For example, in a prospective interventional cohort study involving 53 frail inpatients with a Clinical Frailty Scale score of four or greater, medication reviews were conducted by a Medication Rationalization team involving physicians and led by pharmacists, and compared against 51 control patients who did not receive this review. Furthermore, a randomised controlled trial in a hospital setting demonstrated that 88 percent of deprescribing recommendations based on STOPPFrail were accepted and implemented, with no adverse events reported during a three-month follow-up period. ^[3]

Primary care physicians often face distinct psychological and clinical barriers when attempting to reduce medication burdens. General practitioners typically distinguish between symptomatic medication and preventive medication. While clinicians feel competent managing and stopping symptomatic treatments when conditions are cured or relieved, stopping long-term preventive medication is perceived as more difficult. This hesitation stems from a lack of individualised benefit and risk information for older or frail patients receiving preventive agents. Additionally, some prescribers report ethical dilemmas and a fear of negative outcomes as barriers to deprescribing. ^[4]

Elderly patients presenting with multimorbidity are frequently treated according to clinical guidelines, which results in them using a large number of drugs. This high volume of pharmacotherapy increases the overall risk of adverse drug events, and stopping unnecessary medication may relieve these harmful effects, thereby improving the patient's wellbeing. To facilitate the clinical management of polypharmacy, expert-driven instruments have been developed, though they have demonstrated little effect on patient outcomes, specifically regarding mortality or quality of life. Because quality of life late in life is the most important goal for patients, physicians must focus primarily on patient preferences. ^[4]

Treatment targets and goals of care for the individual patient should be adapted over time according to the preferences of the patient and his life expectancy. However, general practitioners face barriers when exploring patient preferences during medication reviews. Some clinicians believe that confronting a patient with a discussion about life expectancy versus quality of life is unethical, whereas others report that this discussion has a positive effect on the doctor-patient relationship. Furthermore, general practitioners' beliefs concerning elderly patients act as a barrier to exploring patient preferences, and certain patient characteristics can hinder the patient's understanding of the issue. ^[4]

To overcome these barriers, general practitioners would welcome decision support when dealing with several guidelines for one patient, alongside explicit rules for collaborating with medical specialists. Training in shared decision-making helps clinicians elicit patient preferences effectively. ^[4]

When new patients enter the practice list, this is welcomed as an opportunity to review their medication, although clinicians sometimes complain about having an inadequate overview. Intervening in long-term preventive medication is distinct from starting new agents, but evidence shows that antihypertensive agents can be stopped in elderly patients without harm, encouraging clinicians to seriously consider stopping preventive medication when appropriate. Successful deprescribing requires structured training to translate patient preferences into congruent treatment changes, followed by careful monitoring of the effects of this patient-driven medication review on the patient's wellbeing. ^[4]

Rational prescribing requires careful evaluation of drug utilization indicators across clinical encounters to ensure patient safety and optimize therapeutic outcomes. In hospital outpatient settings, investigations into prescribing patterns have documented a mean number of drugs per encounter of 3.51 with a standard deviation of 1.33. Prescribing cascades and inappropriate medication use can be driven by multiple systemic and behavioral factors. A large majority of clinical professionals, specifically 93.8% of surveyed prescribers and dispensers, agreed that polypharmacy may be precipitated by direct patient influence on prescribers. Furthermore, specific professional subgroups hold distinct perceptions regarding prescribing drivers; among dispensers alone, 95.7% agreed that prescribers were profiting from sales of prescribed drugs, 52% believed that clinical diagnoses were not usually accurate, and more than a third, representing 39.1%, believed that prescribers lack the required clinical training. ^[2]

Additional studies evaluating outpatient hospital clinics reveal that about one-fifth, specifically 19% of prescriptions, contained four or more drugs, suggesting a relatively low level of polypharmacy in that particular cohort. The percentage of generic drugs prescribed was found to be 58.5%, while 41.3% of prescriptions had two or more non-generic drugs. Furthermore, the percentage of prescriptions with one or more injections reached 38.7%. A high proportion of drugs prescribed, measured at 96.3%, were sourced from the essential drug list. In contrast, there were surprisingly low percentages of prescriptions featuring antibiotics at 0.33% and analgesics at 2%. High rates of injection use in certain clinical environments can be attributed to specific therapeutic choices, such as the use of depot medications in patients suffering from psychiatric illness. These medications are primarily utilized to improve adherence to treatment regimens, although they are occasionally deployed as adjuncts to oral drugs. There is also a notable frequency of parenteral drug administration in the management of acutely ill patients, especially following a relapse where individuals may present at the outpatient clinic after defaulting from clinic appointments and refusing to comply with oral medications over an extended period. Comparative studies conducted in other international regions such as Nepal, Lagos, and Jordan reported injection use rates of 2.9%, 13.8%, and 15% respectively, highlighting significant regional variability in prescribing practices. ^[2]

To combat irrational prescribing and manage complex multimorbidity, structured tools are deployed across primary and secondary care environments. Pharmacists frequently utilize explicit screening instruments such as the STOPP and Beers criteria, or implicit guidelines of sedatives and anticholinergic medicines developed by research teams, to identify problematic prescribing among frail older people and care home residents.

Rational prescribing requires careful management of polypharmacy, particularly in elderly patients where clinicians must balance therapeutic guidelines against the vulnerability of the individual. When deprescribing is indicated, stopping medicines one at a time is most appropriate for managing people whose health status is stable in out-patient settings. In contrast, concurrent deprescribing of multiple medications may be more appropriate for inpatients where it is easier to monitor for withdrawal effects. When managing very elderly patients with multimorbidity, general practitioners often find themselves balancing between the risks of polypharmacy, the vulnerability of elderly patients, and guidelines that recommend prescribing. This clinical balancing act requires structured review tools to evaluate the ongoing necessity of each prescribed agent.

Clinical trials and systematic reviews demonstrate varying outcomes regarding medication reduction strategies and their impact on patient safety endpoints. A review published in 2017 reported that falls-risk drug withdrawal strategies did not significantly change the rate of falls, number of people who fell or rate of fall-related injuries over a 6 to 12 months follow-up period across five included papers. However, another review suggested that deprescribing interventions could significantly reduce the number of people who fall in care homes by 24%. These discrepancies across studies may stem from the specific tools used during the intervention; algorithms focusing exclusively on sedative and psychotic medications can result in a higher proportion of anticholinergic medications being stopped compared to tools with a broader remit like STOPPFrail. Furthermore, focusing narrowly on specific drug classes can lead to inconsistent findings regarding the relationship between falls and deprescribing in the wider literature. ^{[4] [3]}

Quantitative pharmacology provides the mathematical framework for rational prescribing. The key equations describe drug exposure, dose–response relationships and the time course of drug action. Understanding these allows the prescriber to individualise therapy rather than rely on trial and error.

Worked example 1: Loading dose calculation

A 70 kg patient requires an immediate therapeutic concentration of digoxin of 1.5 µg/L for atrial fibrillation. The volume of distribution is 7 L/kg and oral bioavailability is 0.7.

• Vd = 7 L/kg × 70 kg = 490 L
• LD = (Vd × Cp_target) / F = (490 L × 1.5 µg/L) / 0.7 = 735 µg / 0.7 = 1050 µg = 1.05 mg

Illustrative example: the calculated loading dose is approximately 1 mg, which would be rounded to a clinically practical dose.

Worked example 2: Maintenance dose calculation

The same patient has a digoxin clearance of 100 mL/min (6 L/h). The target average steady-state concentration is 1.5 µg/L, with a dosing interval of 24 hours.

• MD = (CL × Cp_target × τ) / F = (6 L/h × 1.5 µg/L × 24 h) / 0.7 = 216 µg / 0.7 = 309 µg ≈ 0.3 mg daily

Illustrative example: a maintenance dose of 0.25 mg daily would be a reasonable clinical choice.

Worked example 3: Half-life and dosing interval

A drug has a clearance of 2 L/h and a volume of distribution of 40 L.

• t½ = 0.693 × Vd / CL = 0.693 × 40 L / 2 L/h = 13.9 hours

A dosing interval of approximately one half-life (12–14 hours) would be appropriate to maintain relatively stable concentrations. For drugs with a narrow therapeutic index, the interval is often set to one half-life or less; for drugs with a wide therapeutic index, longer intervals may be acceptable for convenience.

Worked example 4: Steady-state concentration prediction

A patient receives 500 mg of a drug every 12 hours. Bioavailability is 0.8 and clearance is 4 L/h.

• Cp_ss = (F × Dose) / (CL × τ) = (0.8 × 500 mg) / (4 L/h × 12 h) = 400 mg / 48 L = 8.33 mg/L

Illustrative example: the predicted average steady-state concentration is approximately 8 mg/L, which can be compared with the therapeutic range to assess adequacy.

Worked example 5: Bioavailability calculation

A drug is given intravenously (AUC = 40 mg·h/L) and orally (AUC = 28 mg·h/L).

• F = AUC_oral / AUC_IV = 28 / 40 = 0.7

The oral bioavailability is 70%, meaning 30% of the oral dose is lost to first-pass metabolism or incomplete absorption.

Worked example 6: Creatinine clearance and dose adjustment

For renally cleared drugs, the Cockcroft–Gault equation estimates creatinine clearance:

• CrCl (mL/min) = [(140 − age) × weight (kg)] / (72 × serum creatinine (mg/dL)) × (0.85 if female)

A 75-year-old woman weighs 60 kg and has a serum creatinine of 1.2 mg/dL.

• CrCl = [(140 − 75) × 60] / (72 × 1.2) × 0.85 = (65 × 60) / 86.4 × 0.85 = 3900 / 86.4 × 0.85 = 45.1 × 0.85 = 38.4 mL/min

Illustrative example: this patient has moderate renal impairment, and doses of renally cleared drugs (e.g., gentamicin, digoxin) would need adjustment, typically by extending the dosing interval or reducing the dose.

The quantitative approach to prescribing is not merely academic—it directly improves patient outcomes. When combined with the principles of rational drug use—generic prescribing, appropriate number of drugs, and avoidance of unnecessary injections—it forms the foundation of safe and effective pharmacotherapy.

Rational drug prescribing becomes critically complex when managing elderly patients with multimorbidity. These vulnerable individuals frequently receive multiple treatments in accordance with professional guidelines for each individual condition. Preventive medications, specifically those prescribed for cardiovascular risk management and the management of diabetes mellitus, contribute a substantial proportion of this total drug burden. Clear information regarding the precise clinical effects of these combined therapies in older populations is often lacking. ^[4]

A major clinical challenge in this population is that the risk reductions established in controlled clinical trials are rarely strictly cumulative. Consequently, the overall clinical benefit experienced by elderly patients is often less than the arithmetic sum of the individual single therapies. Furthermore, managing a high volume of medications places a heavy burden on the patient and significantly enhances the susceptibility to adverse drug events. Patient-reported outcomes, geriatric syndromes e.g. falls, fractures, gait speed, depression and delirium or cost evaluations were infrequently reported, and frailty was not used as either inclusion criteria or an outcome measure When evaluating older populations, research d ^[3][4]

Recent clinical investigations have expanded knowledge by focusing on deprescribing among older people identified as living with frailty. Unlike most published reviews that focused on the general population of older people or specific settings, identifying frailty using reliable measures allows the implementation of effective interventions in a population more vulnerable to harm from medications. However, inclusion criteria in such studies often required a reliable and valid measure of frailty to allow extrapolation of the results to patients with similar scores or to measure whether frailty status affects response to deprescribing interventions. ^[3]

Rational drug prescribing and advanced medical interventions require careful integration of clinical investigations, diagnostic imaging, and laboratory measurements to guide therapeutic decisions safely and accurately. When evaluating tissue structures and planning treatments that involve complex pharmacological or radiation interventions, computed tomography (CT) serves as the primary basis for treatment planning due to its high geometrical accuracy, widespread availability, and direct connection to electron density measurements.

To maintain rigorous precision during diagnostic and planning imaging, CT scanners undergo strict quality assurance protocols. For example, when imaging is performed using a slice thickness of 2.5 mm and 130 kV, verification using a specialized CATPHAN 600 phantom demonstrates that peripheral Hounsfield Unit values vary by less than 4 HU, which corresponds to 0.4% of the attenuation coefficient of water when compared to the center of the phantom. ^[5]

When patient and group-based Hounsfield unit density tables are applied across different anatomical sites, specific dosimetric variations emerge between planning CT and cone-beam CT.

While computed tomography remains fundamental for attenuation mapping, complementary diagnostic modalities such as magnetic resonance imaging add significant clinical value in defining target volumes for brain lesions, prostate targets, and head and neck tumours. Furthermore, cone-beam CT systems show varying performance profiles across different clinical platforms. Investigations of the Varian OBI system reveal small differences in density calibration of less than 10 HU between planning scans and cone-beam CT, whereas the Elekta Synergy system exhibits larger deviations that necessitate specific correction strategies. In specific populations or low-resource settings where advanced multimodality imaging is limited, clinicians must rely carefully on standard calibrations and validated tissue lookup tables to minimize calculation errors and optimize therapeutic safety. ^[6]

Rational design to control pharmacokinetics is particularly vital in the development of drugs targeting the central nervous system, a pharmaceutical sector characterised by some of the poorest success rates in clinical development. For a compound to exert its action within the brain, successful penetration across the blood-brain barrier is mandatory. This permeability can be achieved if the candidate molecule is highly lipophilic, enabling passive diffusion, or if it acts as a specific substrate for an influx transporter. However, successful central nervous system drug delivery faces a strict physiological caveat: the candidate molecule must not simultaneously act as a substrate for efflux transporters, such as the adenosine triphosphate-binding cassette transporters or amino acid transporters. ^[1]

In clinical practice, managing medication in elderly populations requires careful distinction between therapeutic agents and those intended purely for prevention. There is a recognised clinical divergence where doctors and patients may readily accept adverse drug events as unavoidable, yet older patients frequently harbour unvoiced concerns regarding the necessity of taking all concurrent medications. Older individuals often prioritise their overall quality of life above extending life expectancy. Consequently, stopping preventative medications can relieve distressing adverse drug events and substantially enhance patient wellbeing, given their limited life expectancy. To assist clinicians in managing polypharmacy safely, expert-driven instruments include protocols for reviewing medication using explicit criteria encompassing patient characteristics, comorbidities, and pharmacodynamics. ^[4]

Children are not small adults. Drug disposition changes dramatically across paediatric age groups, and dosing is usually based on body weight or body surface area rather than fixed adult doses. Hepatic drug-metabolising enzymes mature at different rates; for example, cytochrome P450 activity is reduced in neonates but can exceed adult levels in older infants and young children, altering both clearance and the risk of toxicity. Renal function also matures postnatally, reaching adult levels by around 1 year of age. Because children are a vulnerable group, off-label prescribing is common but must be justified, and formulations should be appropriate for age (e.g., liquid preparations, dispersible tablets). The risk of adverse drug reactions is heightened by poly-pharmacy, defined as the practice of prescribing four or more medications to the same person. In children with chronic conditions such as epilepsy or asthma, multiple drugs are often needed, increasing the risk of drug–drug interactions and non-adherence. ^[8]

Older adults frequently have multiple chronic diseases, and poly-pharmacy is a major concern. Poly-pharmacy increases the risk of adverse drug reactions, severe drug–drug interactions, non-adherence to prescription and prolonged hospitalization. Age-related physiological changes—reduced renal clearance, decreased hepatic blood flow, altered body composition—prolong drug half-lives and increase sensitivity to many agents (e.g., benzodiazepines, anticholinergics). Prescribers should regularly review medications, deprescribe where possible, and prefer drugs with a wide therapeutic index. The influence of patients on prescribers and the demand from patients can drive irrational prescribing, so prescribers must resist pressure for unnecessary drugs. ^[8]

During pregnancy, physiological changes (increased plasma volume, enhanced renal blood flow, altered hepatic metabolism) change drug pharmacokinetics, often requiring dose adjustments. Teratogenic risk is highest in the first trimester, so non-essential drugs should be avoided. In breastfeeding, many drugs pass into milk; the infant dose depends on the drug's lipid solubility, molecular weight and protein binding. Prescribers should choose agents with a favourable safety profile in lactation and consider the timing of doses relative to feeds. Rational prescribing here means balancing maternal benefit against fetal and infant risk, and documenting the decision clearly.

Renal impairment reduces excretion of renally cleared drugs, so doses must be adjusted according to creatinine clearance (e.g., for aminoglycosides, digoxin, metformin). Hepatic impairment affects drugs that undergo extensive first-pass metabolism or biliary excretion (e.g., warfarin, opioids, many antifungals). In both settings, the prescriber should estimate the degree of impairment, choose drugs that are less dependent on the affected organ, and monitor drug levels or clinical response closely. Failure to adjust doses in organ impairment is a common cause of adverse drug reactions and contributes to irrational drug use.

Genetic polymorphisms in drug-metabolising enzymes (e.g., CYP2D6, CYP2C19) and transporters can cause large inter-individual differences in drug response and toxicity. Poor metabolisers may accumulate drugs to toxic levels, while ultra-rapid metabolisers may fail to achieve therapeutic concentrations. Pharmacogenetic testing can guide dosing for certain drugs (e.g., codeine, warfarin, thiopurines). Additionally, designing drugs that are not sensitive to resistance mutations—binding to drug-resistant variants as well as the wild-type target—is particularly important for rapidly mutating targets in infectious disease (e.g., HIV) and cancer. This broad, promiscuous coverage is a growing priority in rational drug design. ^[7]

When evaluating complex regimens in clinical pharmacology exams, candidates must distinguish between explicit criteria, such as the STOPP criteria, STOPPFrail tool, and Beers criteria, and implicit tools, such as the Garfinkel algorithm for concurrent deprescribing. Explicit criteria provide clear lists of medication names to avoid, whereas implicit tools use evaluative questions to guide clinical judgement. Combined protocols frequently integrate STOPP guidelines, Beers criteria, Choosing Wisely, and pharmacist judgement.

Understanding the mechanisms of drug selectivity provides a solid first-principles foundation for rational drug design and pharmacology. Selectivity can be achieved by designing compounds that fit within a larger binding site while clashing with a smaller site, yielding over 13000-fold selectivity against the smaller binding site. Selectivity against a smaller site primarily exploits the strongly repulsive van der Waals potential at short distances, whereas targeting a larger site relies on the loss of favorable van der Waals interactions, which is a much weaker effect than introducing steric clashes. ^[1]

Protein plasticity and flexibility are crucial when predicting binding affinity and selectivity. Induced fit effects can confound predictions derived solely from static crystal structures, making it vital to understand the plasticity of undesirable decoy structures. Conversely, flexibility can be leveraged to gain selectivity if the target is flexible while the decoy remains rigid. Interfacial water molecules and allosteric modulation also influence binding interactions, providing alternative pathways to achieve high specificity when decoys lack distinct allosteric pockets. ^[1]

Prescribers frequently fall into the trap of polypharmacy without periodically reassessing the original indication for each drug. Relying exclusively on static structural models without accounting for target flexibility or interfacial water molecules leads to major errors in predicting drug binding and selectivity. In computational modeling and structure-based drug design, failing to constrain systems properly during free energy perturbation or thermodynamic integration can allow a water molecule annihilation to create a vacant cavity that another water molecule fills, generating misleading energetic contributions. ^[1]

Another common error is ignoring the limitations of computational enzyme conversions. Although converting kinase structures to the DFG-out form is conceptually useful for virtual screening and structure-based lead optimization, it is difficult to determine whether the resulting structure is energetically accessible in a real biological system without prospective validation. ^[1]

Rational drug prescribing and structured medication reviews are applied daily in geriatric medicine, clinical pharmacology wards, and multidisciplinary primary care clinics. Clinicians use formal deprescribing tools to streamline complex drug regimens, minimize adverse drug reactions, and reduce the burden of anticholinergic and sedative medicines.

In pharmaceutical research and development, rational drug design principles are applied to optimize selectivity profiles against therapeutic targets such as kinases and enzymes. Computational protocols screen candidate molecules against ensembles of desirable target shapes and undesirable decoy shapes, utilizing objective functions to tune selectivity and avoid off-target toxicity. ^[1]