A 15-year-old girl is prescribed a single dose of 1.5 mg levonorgestrel for emergency contraception. She has severe asthma and also takes phenobarbital for epilepsy. What should she do?

• A. Increase the dose of Levonorgestrel
• B. Take with food
• C. No adjustment needed
• D. Use a different emergency contraceptive

Answer: (A)

When a patient is taking an enzyme-inducing anticonvulsant like phenobarbital, drugs that are processed by the liver are cleared more quickly. Levonorgestrel, used for emergency contraception, is affected by this kind of interaction, so a standard single 1.5 mg dose may not achieve the necessary hormone exposure to prevent pregnancy. To counteract that faster metabolism, the dose should be increased to provide enough levonorgestrel despite the induction. That means using a total of 3 mg, typically by giving two 1.5 mg tablets at once, rather than sticking with the usual 1.5 mg single-dose regimen. The goal is to maintain effective levels during the critical window after unprotected intercourse. Taking it with food doesn’t address the reduced levels from enzyme induction, and while alternatives like a copper IUD exist, the recommended adjustment with phenobarbital is to increase the levonorgestrel dose. The asthma itself doesn’t change this interaction.