One in three women with epilepsy on susceptible contraception also takes an enzyme inducer

Design

Retrospective claims cohort

Population

21,923 people with epilepsy prescribed susceptible hormonal contraception

Primary outcome

At least 14 days' overlap with an enzyme-inducing antiseizure medication

Effect

32.6% (7,154); median overlap 218 days (IQR 74-511)

A retrospective study of US Medicaid claims identified 21,923 people with epilepsy of child-bearing potential prescribed a hormonal contraceptive whose effect is reduced by liver enzyme induction. Of these, 7,154 (32.6%) overlapped with an enzyme-inducing antiseizure medication for at least 14 days, and the median overlap was 218 days.

Overlap was more likely in people with intellectual disability (adjusted OR 2.33), drug-resistant epilepsy (1.73) and those over 35 (1.45). The data cannot show whether patients were counselled, used a barrier method as well, or became pregnant.

The interaction is well known; this study shows how often it is missed in routine care. Carbamazepine, phenytoin, phenobarbital, oxcarbazepine and topiramate at higher doses are among the drugs that can reduce the effect of oral, patch, ring and implant contraception. Copper and levonorgestrel intrauterine devices and depot medroxyprogesterone are generally considered unaffected.

• At every review of a woman of reproductive age with epilepsy, ask what contraception she uses.
• Check whether her antiseizure medication induces liver enzymes.
• If it does, discuss intrauterine or depot methods, which the interaction does not affect.
• Take particular care with patients who have intellectual disability or drug-resistant epilepsy, who were most often affected.
• Pair contraception advice with folic acid and pregnancy-planning counselling.

Why it matters

An unplanned pregnancy on an enzyme-inducing antiseizure drug carries risks for both mother and baby that a contraception question could have avoided.

Don't overread it

Claims data show co-prescription, not whether counselling, barrier methods or pregnancies followed.

The statistics, in plain English

The adjusted odds ratios describe which patients were more likely to have an overlap, after accounting for other characteristics. An OR of 2.33 means roughly double the odds; the narrow confidence intervals reflect the very large sample, not the size of any harm.

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