Abstract

PIP: Various drugs are capable of enhancing or inhibiting the effect of oral contraceptives (OCs). Drugs of the rifampicin group are active enzyme inductive agents which affect both the estrogenic and progestagenic components of OCs. This was demonstrated by the following pharmacokinetic study. 8 women received a single dose of Minovlar (50 mg of ethinyl estradiol and 1 mg of norethindrone acetate) during a rifampicin course, and another dose of the same OC was administered after the completion of the drug course. Rifampicin reduced the half-life of ethinyl estradiol from 6.2 to 3.2 hours and that of norethindrone acetate from 6.5 to 2.9 hours. Anticonvulsive drugs, e.g., phenytoin, phenobarbital, methylphenobarbital, and carbamazepine, often cause failures of contraceptive programs using OCs. A recent study followed the effect of phenobarbital in 4 women taking OCs. In two cases, the concentration of estrogen in blood plasma decreased significantly and hemorrhages occurred. In such cases, other methods of contraception should be recommended. Statistics show that, in 1982, 38 English women experienced unsuccessful contraception because they took various antibiotics simultaneously with OCs. This effect was reported for ampicillin, tetracycline, erythromycin, nitrofurantoin, and other antibiotics. On the other hand, ascorbic acid (Vitamin C) enhances the effect of OCs. The above studies prove that the interactions of rifampicin, anticonvulsive drugs, and antibiotics with OCs are pharmacodynamic, rather than pharmacokinetic, interactions. There is no clinical evidence to prove that OCs interfere with the metabolism of certain drugs. Future clinical and paraclinical studies are needed for a better understanding of these effects.