Drug interactions
Fosphenytoin + Norethisterone
Severity: Major
Fosphenytoin and Norethisterone: 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
Fosphenytoin is converted to phenytoin, a potent enzyme inducer that speeds up the metabolism of norethisterone. Contraceptive efficacy can fall, leading to breakthrough bleeding and unplanned pregnancy, and this is of particular concern because of phenytoin teratogenicity.
What this interaction means in plain words
Fosphenytoin and Norethisterone 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; Norethisterone belongs to Endocrine.
Why it happens
Phenytoin induces CYP3A4 and other hepatic enzymes, increasing clearance of norethisterone.
What to do
Use a non-enzyme-affected contraceptive such as a copper IUD or levonorgestrel IUS, or add barrier methods. Continue the additional protection for several weeks after stopping fosphenytoin.
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 Norethisterone
- Acitretin + Norethisterone Contraindicated
- Amobarbital + Norethisterone Major
- Apalutamide + Norethisterone Major
- Belzutifan + Norethisterone Major
- Bosentan + Norethisterone Major
- Brigatinib + Norethisterone Major
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 Norethisterone?
MediHelp rates the interaction as major. Fosphenytoin and Norethisterone 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 is converted to phenytoin, a potent enzyme inducer that speeds up the metabolism of norethisterone. Contraceptive efficacy can fall, leading to breakthrough bleeding and unplanned pregnancy, and this is of particular concern because of phenytoin teratogenicity.
How serious is the interaction between Fosphenytoin and Norethisterone?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Use a non-enzyme-affected contraceptive such as a copper IUD or levonorgestrel IUS, or add barrier methods. Continue the additional protection for several weeks after stopping fosphenytoin.
What other medicines interact with Norethisterone?
MediHelp also lists interactions of Norethisterone with, for example, Acitretin, Amobarbital, Apalutamide. The full list is on the Norethisterone page.
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