Toxic Alcohols: Methanol, Ethylene Glycol and Fomepizole
Here is the strange truth at the heart of this poisoning: the alcohol the patient swallowed is barely toxic. Methanol from windscreen wash and ethylene glycol from antifreeze do little harm as they are — they mostly make a person look drunk. The catastrophe comes hours later, and it is entirely home-grown. The body's own enzyme, alcohol dehydrogenase, mistakes these alcohols for ethanol and diligently converts them into acids that dissolve the optic nerve and crystallise inside the kidneys. The antidote is therefore not something that neutralises the poison — it is a drug that stops your own liver from manufacturing it. Win the race against that enzyme and the patient walks out unharmed; lose it and they leave blind, or on dialysis, or not at all.
A 39-year-old man is brought to the ED at midnight, drowsy and slurring, smelling faintly of alcohol. His partner found an empty bottle of car antifreeze in the garage after an argument. His breath alcohol is negative, which puzzles the triage nurse. For a few hours he looks like nothing more than an ordinary drunk, and improves slightly. Then, near dawn, everything turns: he begins to breathe deep and fast, his blood gas shows a savage metabolic acidosis, and his kidneys stop making urine. A second patient in the next bay drank cheap smuggled spirits at a party; by morning he is complaining that the room has gone hazy, "like looking through a snowstorm," and his pupils barely react. Two different bottles, one shared mechanism — and in both, the clock had been running silently since the first sip.
The poison that isn't the poison
Swallowed as they are, methanol and ethylene glycol are surprisingly gentle. Both are small alcohols, and both do much the same thing that drinking-alcohol (ethanol) does at first: they cross into the brain and cause intoxication — sedation, slurring, unsteadiness. If that were the whole story, these would be minor poisonings. The danger is not the molecule you swallow; it is what your liver turns it into. Alcohol dehydrogenase (ADH), the same enzyme that begins breaking down the ethanol in a glass of wine, cannot tell these impostors apart from its normal substrate. It grabs them and oxidises them into a cascade of small, highly reactive acids. Everything toxic about these poisonings — the blindness, the acidosis, the kidney failure — is the work of those metabolites, not the parent alcohol. This is a pure example of the toxic-metabolite principle introduced in the Principles of Pharmacology chapter.
Methanol → formic acid → blindness
Methanol is found in windscreen washer fluid, some solvents, and illicitly distilled "moonshine" spirits, where it is the notorious cause of mass poisonings. ADH oxidises methanol to formaldehyde, which is almost instantly converted onward to formic acid (formate). Formate is the villain. It poisons cellular respiration — it inhibits the mitochondrial enzyme cytochrome oxidase — and it has a cruel special affinity for the retina and optic nerve. The clinical signature is therefore visual: blurred vision, a described "snowstorm" or field of white, dilated poorly-reactive pupils, and, if untreated, permanent blindness. At the same time the accumulating formic acid drives a severe high-anion-gap metabolic acidosis, and in the worst cases haemorrhagic damage to the basal ganglia of the brain (a putaminal necrosis seen on imaging).
Ethylene glycol → oxalic acid → kidney failure
Ethylene glycol is the sweet-tasting liquid in engine antifreeze and coolant — a taste that makes accidental poisoning of children and pets a real risk. ADH again does the damage, oxidising it in steps: first to glycolaldehyde and glycolic acid — the main driver of the metabolic acidosis — and finally to oxalic acid (oxalate). Oxalate is the metabolite that defines this poisoning. It binds avidly to calcium in the blood, forming insoluble calcium oxalate crystals. Two consequences follow. First, those crystals precipitate in the renal tubules and, together with the direct toxicity of the acids, cause acute kidney injury — the reason the antifreeze patient stops making urine. Second, mopping up calcium as crystals drops the blood calcium, producing hypocalcaemia (which can cause tetany, a prolonged QT interval, and seizures). The tell-tale bedside finding is envelope-shaped or needle-like calcium oxalate crystals in the urine.
Think of alcohol dehydrogenase as an over-eager factory worker on a conveyor belt whose only job is to stamp "process this" on passing alcohols. Ethanol, methanol, ethylene glycol — to him they all look the same, so he stamps them all. With ethanol the finished product is harmless. With the toxic alcohols, the worker is unwittingly assembling a bomb, one benign-looking part at a time, until the finished acid detonates in the eye or the kidney. The clever move is not to defuse the bomb after it is built — it is to distract the worker so he never starts. Fomepizole and ethanol are two ways of keeping him too busy to pick up the poison.
Two gaps, and the trap of timing
The single most important diagnostic idea here is that the labs change over time. Early on — while the parent alcohol is still unmetabolised in the blood — it adds osmotically active particles that the standard sodium/glucose/urea calculation doesn't account for. This produces a raised osmolar gap (a discrepancy between the measured and calculated serum osmolality). At this stage the pH may still be normal and the anion gap unremarkable, because little acid has formed yet. Then, as ADH grinds the alcohol into its acids, the picture flips: the parent alcohol disappears (osmolar gap falls) while the accumulating formate or glycolate drives up the anion gap and the pH crashes. So you may catch a patient at either end — an early osmolar gap with a still-quiet acidosis, or a late roaring anion-gap acidosis with the osmolar gap already resolving. Timing these two gaps against each other, and against the moment of ingestion, is a classic exam and bedside skill; the underlying arithmetic of both gaps is worked through in the Enhanced-elimination & Antidotes chapter.
- Methanol and ethylene glycol are themselves only mildly toxic — the danger is their metabolites.
- Alcohol dehydrogenase (ADH) is the enzyme that manufactures the toxic acids from both.
- Methanol → formaldehyde → formic acid → optic-nerve/retinal toxicity (blindness) + acidosis.
- Ethylene glycol → glycolic acid (acidosis) → oxalic acid → calcium oxalate crystals → AKI + hypocalcaemia.
- Early = raised osmolar gap (parent alcohol); later = rising anion-gap metabolic acidosis (toxic acids).
- Urinary calcium oxalate crystals point to ethylene glycol.
The antidote is to block the enzyme
If the metabolite is the poison, the cure is to stop it being made. Every specific treatment for these poisonings targets alcohol dehydrogenase, and there are two classic ways to disable it. The preferred antidote is fomepizole (4-methylpyrazole), a direct competitive inhibitor of ADH — it binds the enzyme's active site and simply switches off metabolism, so the parent alcohol sits harmlessly in the blood until it can be cleared. Fomepizole is clean, given by intermittent infusion, needs no drunkenness and little monitoring. The older, cheaper antidote is ethanol itself: because ADH prefers ethanol to the toxic alcohols by a wide margin, saturating the enzyme with an ethanol infusion out-competes the poison for the active site — a beautiful, literal example of competitive enzyme inhibition from the Principles of Pharmacology chapter. Ethanol works, but it is far more troublesome: it sedates the patient, needs a target blood level maintained, and causes hypoglycaemia. This dual role of ethanol — a poison in its own chapter, and a life-saving antidote here — is worth pausing on, and is drawn out in the Ethanol chapter.
Fomepizole — competitive ADH inhibitor; first-line; simple, no sedation, minimal monitoring; expensive. Ethanol (IV or oral) — competes with the toxic alcohol for ADH; effective and cheap but causes intoxication, hypoglycaemia and needs level monitoring; used where fomepizole is unavailable. Sodium bicarbonate — corrects the severe acidosis and, for methanol, helps trap formate in a less-toxic ionised form. Cofactors that divert metabolism down safer paths: folinic acid (leucovorin) helps break formate down in methanol poisoning; thiamine and pyridoxine push ethylene glycol metabolism toward non-toxic products. Haemodialysis — removes both the parent alcohol and the toxic acids and corrects acidosis; reserved for severe acidosis, very high levels, visual or renal end-organ damage.
The whole strategy is counter-intuitive: you must treat hardest when the patient looks best. Give ADH blockade during the latent period — while the parent alcohol is still mostly unconverted and the patient is only mildly drunk — and you prevent the toxic acids from ever forming, sparing the eyes and kidneys entirely. Wait until the blindness, the crashing pH, or the anuria appear, and much of that damage is already irreversible; you are now cleaning up acid that has done its work. Early antidote, given on suspicion before symptoms, is the single decision that saves organs. That is exactly why the Approach chapter warns that the lethal latent period is the trap: the patient who looks like they are sobering up may be the one quietly converting a fatal dose.
- The specific antidote is to block ADH — fomepizole (preferred) or ethanol.
- Fomepizole is a clean competitive ADH inhibitor; ethanol competes for the enzyme but sedates and drops glucose.
- Add sodium bicarbonate for the acidosis; give cofactors — folinic acid (methanol), thiamine + pyridoxine (ethylene glycol).
- Haemodialysis clears both parent alcohol and toxic metabolites — for severe acidosis, high levels, or end-organ damage.
- Treat EARLY, on suspicion, during the latent period — before the acids form and blind or scar.
- A normal exam hours after ingestion does not exclude a lethal dose — the poison is still being manufactured.
- Waiting for symptoms or a confirmatory level before starting the antidote. If toxic alcohol is plausible, block ADH first — by the time blindness or acidosis appears, the metabolites have already done their damage.
- Being reassured by a normal or improving exam a few hours in. The latent period is the trap — the patient looks like they are sobering up while the acids are still forming.
- Trusting a normal osmolar gap to rule out poisoning late in the course. By the time the acidosis peaks the parent alcohol may be gone and the osmolar gap normal — the anion gap is what is elevated then.
A man presents 4 hours after drinking antifreeze. He is mildly intoxicated but otherwise well; venous gas shows a near-normal pH with a raised osmolar gap and a not-yet-elevated anion gap. What is the most important immediate step?
- Methanol and ethylene glycol are almost harmless as swallowed — alcohol dehydrogenase (ADH) converts them into the toxic acids that do the damage.
- Methanol → formic acid → blindness + acidosis; ethylene glycol → glycolic/oxalic acid → calcium oxalate crystals, acute kidney injury and hypocalcaemia.
- A latent period is classic: an early raised osmolar gap (parent alcohol) evolves into a rising anion-gap metabolic acidosis (toxic acids) — timing the two gaps guides diagnosis.
- Block ADH EARLY with fomepizole (preferred) or ethanol, add bicarbonate, cofactors (folinic acid; thiamine + pyridoxine) and haemodialysis for severe cases — treating before symptoms is what saves the eyes and kidneys.
- Goldfrank's Toxicologic Emergencies — Toxic alcohols: methanol and ethylene glycol.
- Rang & Dale's Pharmacology — Drug metabolism and toxic metabolites; alcohol dehydrogenase.
- Katzung Basic & Clinical Pharmacology — Alcohols; management of methanol and ethylene glycol poisoning.
- BNF / BNFC — Fomepizole and ethanol in poisoning; sodium bicarbonate.
- UpToDate / TOXBASE — Methanol and ethylene glycol poisoning: diagnosis and treatment.
- EXTRIP Workgroup — Recommendations on extracorporeal treatment (haemodialysis) for methanol and ethylene glycol poisoning.
- Barceloux DG, et al. AACT ad hoc committee practice guidelines on the treatment of methanol and ethylene glycol poisoning.
