Drug interactions
Mestranol + Phenytoin
Severity: Major
Mestranol and Phenytoin: major interaction.
Last updated
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
Phenytoin can lower circulating estrogen exposure from mestranol-containing contraceptives. This may cause breakthrough bleeding and contraceptive failure with unintended pregnancy, which is particularly important because phenytoin is teratogenic.
What this interaction means in plain words
Mestranol 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.
Why it happens
Phenytoin induces hepatic CYP3A4 and other enzymes, accelerating metabolism of the estrogen and its active form ethinylestradiol.
What to do
Advise a non-hormonal or other reliable contraceptive method such as a copper IUD or DMPA instead of relying on the pill. If the pill is continued, add a barrier method and continue it for several weeks after stopping phenytoin.
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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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 Mestranol with Phenytoin?
MediHelp rates the interaction as major. Mestranol 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. Phenytoin can lower circulating estrogen exposure from mestranol-containing contraceptives. This may cause breakthrough bleeding and contraceptive failure with unintended pregnancy, which is particularly important because phenytoin is teratogenic.
How serious is the interaction between Mestranol 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: Advise a non-hormonal or other reliable contraceptive method such as a copper IUD or DMPA instead of relying on the pill. If the pill is continued, add a barrier method and continue it for several weeks after stopping phenytoin.
What other medicines interact with Phenytoin?
MediHelp also lists interactions of Phenytoin with, for example, Artemether / lumefantrine, Bictegravir, Cabotegravir. The full list is on the Phenytoin page.
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