Drug interactions
Fosphenytoin + Levonorgestrel
Severity: Moderate
Fosphenytoin and Levonorgestrel: moderate 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
Fosphenytoin, like phenytoin, induces hepatic enzymes and can lower levonorgestrel exposure. With an intrauterine system most of the effect is local, but contraceptive reliability may still be somewhat reduced, and oral or emergency use is more clearly affected.
What this interaction means in plain words
Fosphenytoin and Levonorgestrel can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken.
Fosphenytoin belongs to Central Nervous System; Levonorgestrel belongs to Endocrine.
Why it happens
Induction of CYP3A4 by phenytoin increases the metabolic clearance of levonorgestrel.
What to do
For oral or emergency levonorgestrel, use a non-hormonal or additional barrier method, or consider a copper IUD for emergency contraception. For an intrauterine system, counsel the patient on possible reduced reliability and consider adding a barrier method.
What to ask your pharmacist
- Should I take them at different times of day?
- Does the dose of either medicine need to change?
- Is there anything I should avoid, such as alcohol or certain foods, while on both?
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Other interactions of Levonorgestrel
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 Levonorgestrel?
MediHelp rates the interaction as moderate. Fosphenytoin and Levonorgestrel can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken. Fosphenytoin, like phenytoin, induces hepatic enzymes and can lower levonorgestrel exposure. With an intrauterine system most of the effect is local, but contraceptive reliability may still be somewhat reduced, and oral or emergency use is more clearly affected.
How serious is the interaction between Fosphenytoin and Levonorgestrel?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: For oral or emergency levonorgestrel, use a non-hormonal or additional barrier method, or consider a copper IUD for emergency contraception. For an intrauterine system, counsel the patient on possible reduced reliability and consider adding a barrier method.
What other medicines interact with Levonorgestrel?
MediHelp also lists interactions of Levonorgestrel with, for example, Amobarbital, Apalutamide, Belzutifan. The full list is on the Levonorgestrel page.
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