The expected failure rate of oral contraception is three times higher (3.1 per 100 woman-years) in WWE taking enzyme-inducing AEDs compared with the normal population. A systematic review of studies on WWE taking combined oral contraceptives reported that the levels of ethinylestradiol
and levonorgestrel were reduced with carbamazepine, oxcarbazepine, felbamate and vigabatrin. There were no changes in the serum levels of hormones in women taking levetiracetam, gabapentin, zonisamide and tiagabine.

In women who are taking enzyme-inducers and choose to use oral contraception, the
contraceptive efficacy may be improved by

• increasing the oestrogen component to 50 micrograms (maximum 70 micrograms)
• reducing the pill-free interval from 7 days to 4 days
• tricycling (taking three packs back to back).
  If WWE taking enzyme-inducing AEDs
  use oral contraceptives (combined hormonal contraception or progestogen-only pills), transdermal patches, vaginal rings or progestogen-only implants- additional barrier contraception should be used.