Particularly now that the lowest effective doses are in use there is little latitude between success and failure if the absorption, distribution and metabolism are distrubed. Any additional drug-taking must be looked at critically lest it reduces efficacy.
Enzyme induction. The rifamycins, rifampicin and rifabutin, are potent inducers of hepatic drug-metabolising enyzmes. The classic example of failure with the combined pill is break-through bleeding and pregnancy in young women being treated with rifampicin for tuberculosis, or meningitis including eradication of the carrier state. The enhanced metabolism of the steroids results in contraceptive failure. Antiepileptics (phenytoin and carbamazepine but not sodium valproate) create a similar risk. Indeed, all drugs that induce metabolising enzymes (see p. 113) whether prescribed or self-administered (alcohol, tobacco smoking) constitute a risk to
17 A popular term that misleads women (see text below).
18 Levonorgestrel prevented 95% of expected pregnancies if it was taken within 24 h, 85% if taken within 48 h and 58% if taken within 72 h (Grimes D et al 1998 Lancet 352: 428^33).
contraceptive efficacy and prescribing should be specifically reviewed for the effect. Pregnancies have occurred in women taking a contraceptive who commence an antiepileptic drug and doctors have been sued (for negligence) successfully in a court of law.
Broad spectrum antimicrobials, e.g. ampicillin, doxycillin, can reduce the efficacy of combined oral contraceptives by diminishing the bacterial flora that metabolise ethinylestradiol in the large bowel and make it available for recycling. Additional contraceptive measures should be taken during a short course of antimicrobial, and for 7 days thereafter. When the course is long, i.e. > 3 weeks the bacteria have time to recover by developing resistance and additional precautions are unnecessary after the first 14 days.
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