Combined hormonal
contraceptives and possibly progestogen-only oral contracep- tives are less reliable during treatment with
phenobarbital. Inter-menstrual break- through bleeding and spotting can take place, and pregnancies have occurred. Controlled studies have shown that
phenobarbital can reduce contraceptive steroid levels. Similarly, emergency hormonal
contraceptives are considered to be less effective in those taking
phenobarbital. Note that
primidone is metabolised to
phenobarbital and therefore 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-inter- acting antiepileptic. For additional advice on the use of
enzyme inducers, such as
phenobarbital, and
contraceptives, see
contraceptives. Reliable contra- ception 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.