[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Fomepizole (4-methylpyrazole)

[Biochemistry](https://durgical.com/en/biochemistry/) · Molecule

# Fomepizole (4-methylpyrazole)

An alcohol dehydrogenase inhibitor: it stops methanol and ethylene glycol from being turned into their toxic acids.

Updated 2026-11-01

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## Key facts

- A potent competitive inhibitor of alcohol dehydrogenase; it also inhibits CYP2E1.
- Antidote of choice in methanol and ethylene glycol poisoning: the unchanged toxin leaves through the kidney, breath or dialysis.
- Compared with ethanol as an antidote: it does not intoxicate, does not cause hypoglycaemia and needs less close level monitoring.
- It does not undo damage already done: it does not remove formic acid or oxalate already formed.

**Formula**
C₄H₆N₂

**PubChem**
[CID 3406](https://pubchem.ncbi.nlm.nih.gov/compound/3406)

## Structure

- A pyrazole ring (five atoms, two nitrogens) with a methyl group at position 4.

## Clinical notes

- It is combined with haemodialysis when there is severe acidosis, visual or kidney damage, or very high levels.
- The CYP2E1 3D structure (3E4E) shows 4-methylpyrazole bound to the haem iron.
- Dose and duration are set by the poison centre.

## Pathways

- [Ethanol metabolism](https://durgical.com/en/biochemistry/pathways/ethanol-metabolism/)

## Related conditions

- [Methanol poisoning](https://durgical.com/en/conditions/methanol-poisoning/)
- [Ethylene glycol poisoning](https://durgical.com/en/conditions/ethylene-glycol-poisoning/)

## Related

- [Alcohol dehydrogenase (ADH)](https://durgical.com/en/biochemistry/enzymes/alcohol-dehydrogenase/)
- [Cytochrome P450 2E1 (CYP2E1, MEOS)](https://durgical.com/en/biochemistry/enzymes/cytochrome-p450-2e1/)
- [Ethanol](https://durgical.com/en/biochemistry/molecules/ethanol/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which enzyme does fomepizole block?

Alcohol dehydrogenase (and to a lesser degree CYP2E1).

## Why is fomepizole not enough once methanol has caused severe acidosis?

Because it does not remove formic acid already formed; haemodialysis is needed.

## What advantage does fomepizole have over ethanol as an antidote?

It causes no intoxication or hypoglycaemia and its effect is more predictable.

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Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [4-Methylpyrazole (CID 3406)](https://pubchem.ncbi.nlm.nih.gov/compound/3406) (PubChem, NCBI, 2026); [RCSB PDB - 3E4E: Human cytochrome P450 2E1 in complex with the inhibitor 4-methylpyrazole](https://www.rcsb.org/structure/3E4E) (RCSB Protein Data Bank, 2026); [Methanol poisoning: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002680.htm) (MedlinePlus, US National Library of Medicine, 2026); [Ethylene glycol poisoning: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000774.htm) (MedlinePlus, US National Library of Medicine, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/molecules/fomepizole/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/molecules/fomepizole/