- Design
- Retrospective claims-based cohort
- Population
- 21,923 US Medicaid beneficiaries with epilepsy on susceptible hormonal contraception
- Primary outcome
- ≥14-day overlap with an enzyme-inducing antiseizure drug
- Effect
- 32.6% overlapped; median 218 days; intellectual disability aOR 2.33 (2.19–2.48)
A Medicaid claims study in Neurology (7 October) followed 110,976 people with epilepsy who could become pregnant. Of 21,923 prescribed a hormonal contraceptive vulnerable to enzyme induction, 32.6% had at least two weeks of overlap with an enzyme-inducing antiseizure drug, and the median overlap was 218 days.
Overlap was more likely in people with intellectual disability (adjusted odds ratio 2.33), drug-resistant epilepsy (1.73) and those over 35 (1.45) — groups for whom an unplanned pregnancy on antiseizure drugs carries particular risk.
This is an old interaction that is still routinely missed. The fix is simple: at every prescription, ask about contraception; prefer a non-inducing antiseizure drug where clinically suitable, or a method unaffected by induction such as a copper or levonorgestrel intrauterine device or depot medroxyprogesterone. Inducers that remain in wide use include carbamazepine, phenytoin and phenobarbital.
- Ask about contraception every time an antiseizure drug is started or changed
- Know the inducers: carbamazepine, phenytoin, phenobarbital, primidone, and oxcarbazepine or topiramate at higher doses
- Combined pills, progestogen-only pills and implants are affected; IUDs and depot injections are not
- Take extra care in people with intellectual disability or drug-resistant epilepsy
- Document the contraceptive plan in the clinic letter
Why it matters
A well-known interaction still affects a third of people at risk, for months at a time.
Don't overread it
Claims data show co-prescription, not pregnancies or counselling — some patients may have used additional methods.
The statistics, in plain English
The adjusted odds ratio of 2.33 (95% CI 2.19 to 2.48) for intellectual disability is precise because the cohort is very large; it means these patients had more than twice the odds of an interacting overlap.
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