Drug interactions
Isoniazid + Phenytoin
Severity: Major
Isoniazid and Phenytoin: major interaction.
Last updated Sources: US FDA label
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Summary
Isoniazid inhibits phenytoin metabolism, which can raise phenytoin concentrations and cause toxicity such as nystagmus, ataxia and confusion. The effect is greatest in slow acetylators and may appear within days to weeks.
What this interaction means in plain words
Isoniazid and Phenytoin can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for.
Isoniazid belongs to Anti Bacterial; Phenytoin belongs to Central Nervous System.
Why it happens
Isoniazid inhibits CYP2C9 and CYP2C19-mediated hepatic metabolism of phenytoin.
What to do
Measure phenytoin concentrations before and during isoniazid therapy and reduce the phenytoin dose as needed. Monitor for signs of toxicity and for hepatotoxicity.
What to ask your pharmacist
- Does the benefit of taking both outweigh the risk in my case?
- Do I need any blood test, ECG or other check while on both?
- What early warning signs should I watch for, and what should I do if they appear?
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References
Other interactions of Isoniazid
Other interactions of Phenytoin
- Artemether / lumefantrine + Phenytoin Contraindicated
- Bictegravir + Phenytoin Contraindicated
- Cabotegravir + Phenytoin Contraindicated
- Doravirine + Phenytoin Contraindicated
- Elbasvir / grazoprevir + Phenytoin Contraindicated
- Elvitegravir / cobicistat / emtricitabine / tenofovir df + Phenytoin Contraindicated
Frequently asked questions
Can I take Isoniazid with Phenytoin?
MediHelp rates the interaction as major. Isoniazid and Phenytoin can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for. Isoniazid inhibits phenytoin metabolism, which can raise phenytoin concentrations and cause toxicity such as nystagmus, ataxia and confusion. The effect is greatest in slow acetylators and may appear within days to weeks.
How serious is the interaction between Isoniazid and Phenytoin?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Measure phenytoin concentrations before and during isoniazid therapy and reduce the phenytoin dose as needed. Monitor for signs of toxicity and for hepatotoxicity.
What other medicines interact with Isoniazid?
MediHelp also lists interactions of Isoniazid with, for example, Bacillus calmette-guerin substrain tice, Benzhydrocodone / acetaminophen, Carbamazepine. The full list is on the Isoniazid page.
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