Fomepizole (4-methylpyrazole) is a competitive ADH inhibitor. Fomepizole has been shown in vitro to block alcohol dehydrogenase enzyme activity in dog, monkey and human liver. Fomepizole is indicated as an antidote for ethylene glycol (such as antifreeze) or methanol poisoning, or for use in suspected ethylene glycol or methanol ingestion, either alone or in combination with hemodialysis. It should be given when a known or suspected toxic ethylene glycol or methanol ingestion has occurred and the patient has metabolic acidosis and elevated osmolar gap. The most frequent adverse events reported as drug-related or unknown relationship were headache (14%), nausea (11%), and dizziness, increased drowsiness, and bad taste/metallic taste. Reciprocal interactions may occur with concomitant use of fomepizole and drugs that increase or inhibit the cytochrome P450 system (e.g. phenytoin, carbamazepine, cimetidine, ketoconazole). Fomepizole has been shown to induce the expression of CYP2E1 and to inhibit its activity. These effects were enhanced in rats that had been exposed to ethanol. Fomepizole may also inhibit other CYP enzymes and therefore may alter the exposure to other drugs that are metabolised by CYP enzymes.