Drug interactions
Fosphenytoin + Isoniazid
Severity: Major
Fosphenytoin and Isoniazid: 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 can raise phenytoin concentrations, which are derived from fosphenytoin, and cause phenytoin toxicity such as nystagmus, ataxia and confusion. The risk is greater in slow acetylators and in patients with marginal phenytoin clearance.
What this interaction means in plain words
Fosphenytoin and Isoniazid 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.
Fosphenytoin belongs to Central Nervous System; Isoniazid belongs to Anti Bacterial.
Why it happens
Isoniazid inhibits CYP2C9/CYP2C19-mediated phenytoin metabolism, increasing phenytoin levels.
What to do
Monitor phenytoin serum levels and clinical signs of toxicity when starting or stopping isoniazid; reduce the fosphenytoin dose as needed.
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?
Pro+ members
Unlock the full page
Create an account and start a Pro+ trial to unlock the full management advice for Fosphenytoin and Isoniazid, the additional notes, the evidence detail and the whole-prescription interaction checker.
- Full clinical management advice
- Additional notes
- Evidence detail
- Interaction checker for a whole prescription
Try Pro+ free for 7 days on the web. No card needed.
References
Other interactions of Isoniazid
Other interactions of Fosphenytoin
- Dronedarone + Fosphenytoin Contraindicated
- Fosphenytoin + Ledipasvir-sofosbuvir Contraindicated
- Fosphenytoin + Lorlatinib Contraindicated
- Fosphenytoin + Lurasidone Contraindicated
- Fosphenytoin + Ranolazine Contraindicated
- Fosphenytoin + Sofosbuvir-velpatasvir-voxilaprevir Contraindicated
Frequently asked questions
Can I take Fosphenytoin with Isoniazid?
MediHelp rates the interaction as major. Fosphenytoin and Isoniazid 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 can raise phenytoin concentrations, which are derived from fosphenytoin, and cause phenytoin toxicity such as nystagmus, ataxia and confusion. The risk is greater in slow acetylators and in patients with marginal phenytoin clearance.
How serious is the interaction between Fosphenytoin and Isoniazid?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Monitor phenytoin serum levels and clinical signs of toxicity when starting or stopping isoniazid; reduce the fosphenytoin dose as needed.
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.
Full monographs, dosing and an interaction checker in the MediHelp app
Open the complete reference, check multi-drug interactions and use dosing tools in the MediHelp app.
