Specific toxicities and their antidotes

Toxicology is the study of the adverse effects of chemicals on living organisms. In clinical practice, poisoning and overdose are common presentations, and the management of these patients relies on a combination of supportive care, decontamination, and the use of specific antidotes. An antidote is a substance that can counteract, neutralise, or reverse the effects of a poison. The choice of antidote depends on the specific toxin, the severity of the poisoning, and the clinical status of the patient.

Two of the most clinically significant poisonings are those caused by ethylene glycol (EG) and methanol. These are relatively uncommon but remain important causes of suicide and epidemic poisonings, resulting in multiple deaths and serious sequelae. In 2008, the American Association of Poison Control Centers Toxic Exposure Surveillance System reported 922 cases of EG and 825 cases of methanol exposures resulting in a total of 20 deaths. However, this undoubtedly underestimated the real number of cases. Poisonings may occur through self-harm, misuse, or potentially malicious ingestions. ^[2]

The toxicity of both alcohols is related to the production of toxic metabolites by alcohol dehydrogenase (ADH) and aldehyde dehydrogenase, resulting in anion gap metabolic acidosis as well as specific organ injuries. In EG poisoning, calcium oxalate, derived from successive oxidations, may precipitate in tissues, mainly in the renal tubules, resulting in acute renal failure. In methanol poisoning, the toxic metabolite formic acid is responsible for the characteristic visual disturbances and severe acidosis. ^[2]

The management of toxic alcohol poisoning involves two key strategies: the use of antidotes to block ADH metabolism, and dialysis to remove the alcohol and its toxic metabolites. The antidotes used are either a competitive ADH substrate (ethanol) or an inhibitor (fomepizole). Dialysis is used to remove the alcohol and its toxic metabolites, to correct acidosis, and, in the case of methanol poisonings, to shorten the course of hospitalization. ^[2]

• Hemolytic toxicity – destruction of red blood cells, often seen after envenomation by Naja naja karachiensis (Pakistani cobra). The venom destabilises the red‑cell membrane through phospholipase A₂, proteases and 5′‑nucleotidases, producing haemolysis.
• Carbon‑monoxide (CO) poisoning – CO binds haemoglobin with an affinity >200 times that of oxygen, forming carboxyhemoglobin (CO‑Hb) and impairing oxygen delivery. Inhalation of 2 % CO in air converts about 60 % of haemoglobin to CO‑Hb within roughly 2 minutes, a level usually considered lethal.
• Cyanide poisoning – cyanide binds the ferric (Fe³⁺) centre of cytochrome c oxidase, halting cellular respiration. Induction of methemoglobin (Fe³⁺‑containing haemoglobin) can sequester cyanide, freeing cytochrome c oxidase.

• CO exposure: 2 % CO in inspired air produces a lethal CO‑Hb level (~60 %) in about 2 minutes. This rapid rise defines the emergency threshold for initiating high‑flow oxygen therapy. ^[6]
• Methemoglobin formation: Amyl nitrite typically yields < 6 % methemoglobin; this modest rise is sufficient to bind cyanide without causing clinically significant methemoglobinaemia.
• Antidote dosing considerations: While exact milligram doses are not provided in the source material, the therapeutic principle is to use the minimal effective amount that achieves the desired biochemical effect (e.g., enough nitrite to raise methemoglobin modestly, enough onion extract to neutralise venom enzymes).

• Onion (Allium cepa) as a venom antidote – Bulb extracts have been shown to neutralise haemolysis caused by cobra venom, likely by inhibiting phospholipase A₂ and protease activity. ^[5]
• Amyl nitrite inhalation – Historically used as first‑aid for cyanide exposure; produces a rapid but limited methemoglobin rise (< 6 %).
• Sodium nitrite infusion – More commonly employed worldwide for cyanide poisoning; however, clinicians must monitor for severe hypotension, a recognised adverse effect. ^[6]

• CO poisoning cannot be ruled out by normal pulse‑oximetry or PaO₂ values; specific CO‑oximeters are required to detect CO‑Hb.
• Methemoglobin‑forming agents must be balanced against the risk of inducing methemoglobinaemia; the target is a modest increase that sequesters cyanide without compromising oxygen transport.
• Natural products such as onion may offer adjunctive protection against specific toxins (e.g., snake venom) but should not replace established antivenom therapy.

The cornerstone of treatment for EG and methanol poisoning is the administration of an antidote that blocks ADH. Fomepizole is a competitive inhibitor of ADH and is the preferred agent in most settings. Ethanol, which acts as a competitive substrate for ADH, is an alternative when fomepizole is unavailable or contraindicated.

The choice between fomepizole and ethanol depends on several factors. Hospitals seeing only occasionally EG or methanol poisonings and hospitals with easily available hemodialysis may prefer ethanol antidote therapy. However, ethanol combined with hemodialysis should be administered only when fomepizole is unavailable or contraindicated. In the US, despite the advantages of fomepizole, treatments with it still suffer from significant delays and it is used less frequently than recommended by poison center staff. ^[2]

In addition to antidotes, supportive care is essential. Poisoned patients should be admitted to a medical ward and, in case of life-threatening situation, to an intensive care unit. Sodium bicarbonate is required in case of severe academia (pH < 7.3). Hemodialysis, mechanical ventilation, fluids, and vasopressors may be indicated in severe poisonings. ^[2]

Calcium salts are only recommended in EG poisoning if hypocalcemia significantly contributes to symptoms (muscle spasms or seizures), due to the concern about precipitating calcium oxalate crystals. Other adjunctive treatments have been suggested, however with limited demonstration of benefit: pyridoxine in EG poisoning as cofactor of glycolic acid metabolism to glycine; and folinic acid in methanol poisoning as cofactor of formic acid metabolism to carbon dioxide by tetrahydrofolate synthetase due to the small folate pool in humans. ^[2]

The current criteria for hemodialysis in methanol poisonings include severe metabolic acidosis, renal failure, electrolyte disturbance unresponsive to conventional therapy, visual symptoms, deteriorating vital signs despite intensive care, and plasma methanol concentration ≥50 mg/dL (15.6 mmol/L). ^[2]

Prolonged fomepizole administration may obviate the need to dialyze poisonings involving high methanol concentrations (≥50 mg/dL and far above, up to 146 mg/dL in one successfully treated case), without severe acidosis or visual impairment. Delayed hemodialysis or even no hemodialysis may be an option in selected cases. ^[2]

Beyond toxic alcohols, there is growing interest in natural compounds with antidotal properties. Onion (Allium cepa) and its main components have shown protective effects against natural and chemical toxicities through in-vitro and in-vivo studies. Onion has plentiful chemical compounds such as allicin, quercetin, fisetin, other sulphurous compounds: diallyl disulphide and diallyl trisulphide. These components have demonstrated benefits including free-radical scavenging and antioxidant properties, anticholesterolemic, anti-heavy metals toxicity, antihyperuricemia, antimicrobial, anti-gastric ulcer, and anticancer effects. However, these findings are primarily from in-vitro and animal studies, and their clinical applicability in human poisonings remains to be established.

The clinical presentation of toxicological emergencies varies significantly depending on the agent involved, requiring specific antidotal strategies. In cases of ethylene glycol poisoning, leucocytosis is a consistent and usually severe finding, observed in all 17 patients in a specific review of records^[1]. While not specific to this toxin, the universal presence of leucocytosis serves as a useful diagnostic tool; its absence helps rule out ethylene glycol poisoning^[1]. The mechanism may relate to tissue deposits of calcium oxalate crystals, though this remains speculative^[1]. ^[4]

Methanol poisoning presents a different challenge, often with an unclear history that necessitates high clinical suspicion. A patient with normal acid–base status shortly after ingestion should be observed for at least 12 hours, with serial basic metabolic panels performed every 2 to 4 hours to monitor for metabolic acidosis and an elevated anion gap. This 12-hour observation period is the standard of care, based on clinical experience that acidosis typically develops within this timeframe. Diagnosis relies on identifying metabolic acidosis, elevated anion and osmolal gaps, and confirming methanol and formate levels. Management includes securing the airway, addressing acidosis with sodium bicarbonate, and administering antidotes such as fomepizole or ethanol. Fomepizole offers a broader therapeutic index and more consistent pharmacokinetics than ethanol, with fewer side effects. However, its primary drawback is high cost, though savings are achieved by reducing the need for hemodialysis and intensive care admissions. Adverse effects of fomepizole include headache in 12% of cases, nausea in 11%, and dizziness in 7%. No significant interactions with other pharmaceuticals have been reported, but contraindication exists for allergy to pyrazole derivatives. ^{[3] [2]}

Salicylate poisoning presents with fever, which is explained by excessive glucose utilization creating heat, despite aspirin being an antipyretic^[1]. In contrast, envenomation by the Pakistani cobra (N. n. karachiensis) causes hypotension, paralysis, necrosis, and coagulopathy. The venom destabilizes the human red blood cell membrane, causing hemolysis via phospholipase A2 and protease enzymes. While traditional agents like onion bulbs have shown protective effects in experimental models, clinical trials on humans are proposed to verify these impacts. ^[4]

In toxicology, investigations serve two linked purposes: confirming the diagnosis and guiding the timing and intensity of treatment. For most specific poisonings, treatment decisions rest on clinical suspicion combined with readily available laboratory data rather than waiting for confirmatory results. ^[3]

Methanol and ethylene glycol. The key laboratory findings in methanol poisoning are metabolic acidosis with a raised anion gap and an elevated osmolal gap, reflecting the presence of the parent alcohol. Blood methanol concentrations guide management, though treatment must begin on suspicion before levels return. Methanol elimination normally follows zero-order kinetics because alcohol dehydrogenase (ADH) is saturated, with elimination rates ranging from 2.7 mmol/L/h (8.5 mg/dL/h) to 6.3 mmol/L/h (20 mg/dL/h). Once an antidote inhibits ADH, elimination becomes first-order with a half-life extending from 22 to 87 hours. The minimum effective plasma concentration of fomepizole, derived from studies of complete inhibition of formate accumulation in methanol-poisoned monkeys, is 10 μmol/L (0.8 μg/mL). Fomepizole has a volume of distribution of 0.6–1.0 L/kg, low protein binding, and is eliminated by saturable hepatic metabolism with a Km of 0.94 μmol/L—a concentration always markedly exceeded during therapeutic use. Its only active metabolite, 4-hydroxymethylpyrazole, has approximately one-third the potency of the parent compound. ^{[3] [2]}

Salicylates. Salicylate poisoning is characterised by uncoupling of oxidative phosphorylation, which occurs at serum concentrations between 483.4 and 773.5 mg/L. Laboratory assessment includes serum salicylate levels, blood gas analysis (respiratory alkalosis with compensatory metabolic acidosis), and glucose, since extensive uncoupling increases glucose consumption, particularly in the brain. ^[4]

Other toxins. For cadmium, investigations focus on renal and hepatic injury—serum creatinine, urinalysis for proteinuria, and liver enzymes—reflecting its toxic effects on the kidney and liver. Formaldehyde exposure may cause raised blood creatinine and urea with reduced kidney volume and glomerular number in animal models, so renal function tests are relevant. For Clostridium difficile infection, diagnosis is by stool toxin testing for toxin A and toxin B, with binary toxin also recognised though its virulence is not fully known.

Methanol poisoning is a global health concern that contributes to significant morbidity and mortality worldwide. The scale of the problem is substantial: global consumption of methanol exceeds 225 million litres per day, and in recent years, outbreaks in many developed countries have increased alarmingly. Methanol is found in common household and industrial products including windshield washer fluid, gas line antifreeze, carburetor cleaner, copy machine fluids, perfumes, food warming fuels, and other types of fuel. ^[3]

The clinical manifestations of methanol toxicity vary between adult and paediatric populations and between acute and chronic exposure. Adults typically present with gastrointestinal and neurological symptoms, whereas paediatric patients often exhibit more severe outcomes due to differences in metabolism and body weight. Although uncommon among paediatric patients, methanol toxicity has been reported with varying severity. ^[3]

Acute poisoning generally involves a large, single ingestion, leading to rapid symptom onset, whereas chronic poisoning results from small, repeated exposures, causing insidious, progressive symptoms. ^[3]

In mild cases, there is a minimal decrease in central nervous system (CNS) activity, accompanied by weakness, dizziness, and nausea. Severe cases lead to metabolic acidosis, which manifests as blurred vision, bilateral mydriasis, gastrointestinal symptoms, disorientation, visual abnormalities, photophobia, and sporadic blindness. ^[3]

Systemic toxicity includes metabolic acidosis, increased respiratory rate (hyperpnea), loss of appetite, headache, and nausea. Hyperventilation may lead to the initial complaint of shortness of breath in many patients, and some individuals experience chest pain and may be initially diagnosed with acute myocardial infarction. ^[3]

Methanol poisoning is not limited to the CNS; it can impact multiple other organ systems. The cardiovascular system may experience haemodynamic instability, and acute kidney injury may occur because of the systemic influence of acidosis and the accumulation of toxic metabolites. The consequences of untreated poisoning include permanent neurological damage, blindness, and even death. ^[3]

Suspicion of methanol poisoning is based on a history of exposure, physical examination, and blood chemistry tests including arterial blood gases. An elevated anion gap metabolic acidosis (anion gap = (Na⁺ + K⁺) − (HCO₃⁻ + Cl⁻), normal < 17 mmol/L) reflects the accumulation of formate in methanol poisoning. Lactate accumulation can be expected in the most severe cases due to inhibition of oxidative metabolism in the mitochondria, and the lactate level can be subtracted in a 1:1 manner from the anion gap. Measurement of methanol concentrations in plasma (usually using gas chromatography) confirms the diagnosis, but these assays are not rapidly available in the majority of institutions. ^[2]

Timely intervention is crucial when managing methanol poisoning cases. Antidotes such as ethanol or fomepizole are administered to inhibit the activity of alcohol dehydrogenase (ADH), effectively halting the further breakdown of methanol. This intervention reduces the formation of toxic metabolites and mitigates acidosis, CNS effects, and multiorgan dysfunction. ^[3]

The lethal dose of ethylene glycol is considered to be about 100 mL. Since its specific gravity is 1.246, this equals about 124 g or approximately 12,414 mg/dL. Conversion of merely 2% of this amount would chelate the entire calcium pool of the extracellular fluid compartment, which contains about 1,600 mg of calcium for a person with an extracellular volume of 16 L at a plasma calcium of 10 mg/dL (2.5 mmol/L). This illustrates why oxalate-mediated hypocalcaemia is a feared complication in ethylene glycol poisoning, even though the oxalate pathway is quantitatively minor compared to metabolism via glycine. ^[4]

Toxicology questions in exams reward a systematic approach: identify the poison, understand its toxic metabolite, and recall the specific antidote, its dose, and its indications. The single most important concept is that many antidotes do not remove the poison itself but block its conversion to a more toxic metabolite. This is beautifully illustrated by fomepizole and ethanol in alcohol poisoning.

Fomepizole is a competitive inhibitor of alcohol dehydrogenase (ADH). By blocking this enzyme, it prevents the metabolism of methanol and ethylene glycol (EG) into their toxic metabolites—formic acid and oxalic acid, respectively. The clinical consequence is profound: administered early, fomepizole prevents EG-related renal failure and methanol-related visual and neurological injuries. When given before significant acidosis or organ injury develops, fomepizole may even obviate the need for hemodialysis. This is a favourite exam point: the antidote is most effective when given early, before metabolites accumulate. ^[2]

The indications for starting ADH inhibitor therapy are explicitly defined and frequently tested. Therapy is indicated if the serum methanol level is greater than 20 mg/dL, if there is a recent history of ingesting toxic amounts of methanol with a serum osmol gap greater than 10, or if there is strong clinical suspicion of methanol toxicity with at least two of the following criteria: arterial pH less than 7.3, serum bicarbonate less than 20 meq/L (mmol/L), an osmol gap greater than 10, and the presence of urinary oxalate crystals. Note that these criteria are for methanol; in practice, similar thresholds apply to ethylene glycol, and the same ADH inhibitors are used. ^[3]

The dosing regimen for fomepizole is a classic exam question. The recommended regimen is a loading dose of 15 mg/kg IV, followed by four maintenance doses of 10 mg/kg IV given 12 hours apart, and then 15 mg/kg every 12 hours thereafter. If hemodialysis is initiated, the dosing frequency must be increased to every 4 hours because fomepizole itself is dialysable. Therapy should continue until the diagnosis of toxic alcohol ingestion is ruled out, or until blood pH is controlled and serum alcohol concentration has decreased to less than 20 mg/dL. When dialysis is indicated, a continuous infusion of 1 mg/kg/h should be provided to compensate for its elimination. ^[3]

Fomepizole is preferred over ethanol as the first-line antidote for EG and methanol intoxications. It has fewer and less serious side-effects than ethanol, is safe in the paediatric population, but is not recommended during pregnancy. It is contraindicated in patients allergic to pyrazoles. ^{[2] [3]}

A frequent misconception is that fomepizole or ethanol removes the toxic alcohol from the body. They do not—they merely halt its metabolism. The parent compound (methanol or EG) remains in the body with a very long half-life of 43 hours when ADH is inhibited. This explains why hemodialysis remains the definitive method of removal, and why forced diuresis is a poor substitute. Even excreting 10 L of urine per day in a person with 40 L of total body water would remove only about 25% of the methanol in 24 hours. This is a useful illustrative calculation: the volume of distribution is large, and renal excretion is limited. ^[4]

Another pitfall is forgetting that methanol has a much higher vapour pressure than water. At 37°C, the vapour pressure of water is 47 mm Hg, while that of methanol is 230.5 mm Hg—about 5 times that of water. This is the basis for the theoretical approach of increasing insensible loss by sauna bath or exercise, though this is not a practical clinical strategy. ^[4]

Students often confuse the toxic metabolites of different alcohols. Methanol is metabolised to formic acid, causing visual and neurological injury. Ethylene glycol is metabolised to oxalic acid via glyoxylate, with the conversion of glyoxylate to oxalate catalysed by lactate dehydrogenase (LDH). Oxalate is clinically relevant because calcium oxalate crystals in urine are almost universal in EG poisoning, and deposits in tissues including the kidney are implicated in the toxic effects. Oxalate over-production also explains the hypocalcaemia that is very common in EG poisoning. However, a common pitfall is assuming oxalate is the major metabolic product quantitatively—it is actually a minor byproduct, even though it is diagnostically and clinically important. ^[4]

A third pitfall concerns the osmol gap and anion gap. In early toxic alcohol poisoning, the osmol gap is elevated (due to the parent alcohol), while the anion gap may still be normal. Later, as metabolites accumulate, the anion gap rises and the osmol gap falls. Initiating therapy based on a single gap measurement can be misleading. The criteria for initiating therapy include an osmol gap greater than 10, but this must be interpreted alongside clinical suspicion and other metabolic derangements.

Finally, students sometimes forget that fomepizole dosing must be adjusted during hemodialysis. The frequency increases to every 4 hours, or a continuous infusion of 1 mg/kg/h is used. Failing to adjust the dose during dialysis can lead to subtherapeutic levels and continued toxicity. ^{[2] [3]}

In clinical practice, the management of toxic alcohol poisoning follows a clear pathway. The first step is recognition: a patient with metabolic acidosis, an elevated osmol gap, and a history of possible ingestion. The second step is immediate administration of an ADH inhibitor—fomepizole is preferred over ethanol. The third step is assessment for hemodialysis.

In practice, the decision to start fomepizole is often made on clinical grounds before the methanol level returns. The criteria are explicit: serum methanol greater than 20 mg/dL, recent ingestion with osmol gap greater than 10, or strong suspicion with at least two of: pH less than 7.3, bicarbonate less than 20 meq/L, osmol gap greater than 10, or urinary oxalate crystals. In real emergency departments, the presence of urinary oxalate crystals is a rapid bedside clue for ethylene glycol, though it is also listed among the methanol criteria.

Hemodialysis is the definitive method to remove methanol and EG. It is indicated when there is significant acidosis, renal failure, or high alcohol levels. During dialysis, fomepizole dosing must be adjusted because the drug is removed by dialysis. Therapy continues until the diagnosis is ruled out or the serum alcohol concentration is less than 20 mg/dL with controlled pH.

In practice, the use of fomepizole has largely replaced ethanol because of its superior safety profile, predictable pharmacokinetics, and ease of dosing. It is effective in children and adults, though it is avoided in pregnancy. The key practical message is to act early, use the correct dose, and adjust for dialysis.