Fomepizole
Definition
4-methylpyrazole. A competitive inhibitor of alcohol dehydrogenase used as the first-line antidote for methanol poisoning. Ethanol (oral or IV) is the older, cheaper alternative because it out-competes methanol for the same enzyme.
- Chemical name
- 4-methylpyrazole
- Mechanism
- Competitive inhibitor of alcohol dehydrogenase
- Indication
- First-line antidote for methanol poisoning
- Alternative
- Ethanol, older and cheaper, competes for the same enzyme
Common questions
Why does blocking alcohol dehydrogenase help treat methanol poisoning if methanol itself is not the most toxic part?+
Methanol is metabolised by alcohol dehydrogenase into formaldehyde and then formic acid, and it is those metabolites that cause the characteristic blindness and metabolic acidosis. Blocking the enzyme prevents that conversion, allowing the unmetabolised methanol to instead be cleared by the kidneys and lungs.
Why would a clinician choose ethanol over fomepizole despite fomepizole being first-line?+
Fomepizole is expensive and not available in every hospital or region, whereas ethanol is cheap, widely available, and works through the same competitive-inhibition mechanism, making it a practical substitute in resource-limited or emergency settings even though it requires more intensive monitoring.
Does giving fomepizole eliminate the need for other treatment in severe methanol poisoning?+
No. In cases with significant acidosis, high methanol levels, or organ involvement, fomepizole is typically combined with haemodialysis to actively remove methanol and its toxic metabolites, since blocking further metabolism alone does not clear what has already accumulated.
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