Fomepizole, also known as 4-methylpyrazole, is a medication used to treat methanol and ethylene glycol poisoning. It may be used alone or together with hemodialysis. It is given by injection into a vein.

Common side effects include headache, nausea, sleepiness, and unsteadiness. It is unclear if use during pregnancy causes risk to a fetus. Fomepizole works by blocking the enzyme that converts methanol and ethylene glycol to their toxic breakdown products.

Fomepizole was approved for medical use in the United States in 1997. It is on the World Health Organization's List of Essential Medicines.

Fomepizole is used to treat ethylene glycol and methanol poisoning. It acts to inhibit the conversion of these alcohols into their respective aldehydes by alcohol dehydrogenase. The prevents further conversion to the more active toxic metabolites oxalic acid and formic acid, respectively.

Fomepizole is most effective when given soon after ingestion of ethylene glycol or methanol. Delaying its administration allows for the generation of harmful metabolites.

Common side effects associated with fomepizole use include headache and nausea.

Concurrent use with ethanol is contraindicated because fomepizole is known to prolong the half-life of ethanol via inhibiting its metabolism to acetaldehyde by alcohol dehydrogenase.

Fomepizole is a competitive inhibitor of the enzyme alcohol dehydrogenase, found in the liver. This enzyme plays a key role in the metabolism of ethylene glycol, and of methanol.