Combined hormonal
contraceptives and possibly progestogen-only oral contracep- tives are less reliable during treatment with
phenytoin. Inter-menstrual breakthrough bleeding and spotting can take place, and pregnancies have occurred. Controlled studies have shown that
phenytoin can reduce contraceptive steroid levels. Similarly, emergency hormonal
contraceptives are considered to be less effective in those taking
phenytoin. Fosphenytoin, a prodrug of
phenytoin, may interact similarly.
The increase in failure rate appears small, but because of the consequences of an
unwanted pregnancy, especially with drugs that may cause foetal abnormalities,
adjustments should be made. If possible, change to another suitable,
non-interacting antiepileptic. For general advice on the use of
enzyme inducers,
such as
phenytoin, and
contraceptives, see
contraceptives. Reliable
contraception in most patients is said to be achievable with 80 to 100 micrograms
of ethinylestradiol daily. Additional barrier methods are considered the most
appropriate option for the short-term use of enzyme-inducing antiepileptics.