Contraception by Norplant subdermal capsules is not reliable in epileptic patients on anticonvulsant treatment.

The contraceptive efficacy of progestin-only contraception was studied in epileptic patients using NORPLANT subdermal capsules. The effect of anticonvulsants on levonorgestrel plasma levels was determined. NORPLANT subdermal capsules were inserted into nine epileptic women, and ten control women using no medication. Venous blood samples were taken at 0, 1, 3, 6, 9 and 12 months after insertion and the concentration of levonorgestrel was determined by radioimmunoassay. At 3 to 12 months, the overall mean concentration of plasma levonorgestrel was significantly lower in the six epileptics taking phenytoin alone or in combination with other anticonvulsants (203 +/- 128 pg/ml, mean +/- SD) than in the controls (325 +/- 135 pg/ml, p less than 0.01). After one year, nine of the control patients continued the use of NORPLANT and no pregnancies occurred. Two of the nine epileptics became pregnant during contraception by NORPLANT. They both used phenytoin and their plasma concentrations of levonorgestrel were low near the time of conception. Levonorgestrel released from the capsules had no apparent harmful effects on epilepsy and none of the patients reported an increase in seizure frequency. The results show that contraception by the progestin levonorgestrel is not reliable in epileptic patients using anticonvulsants known to induce metabolizing enzymes of the liver.