The interaction of phenobarbital and other anticonvulsants with oral contraceptive steroid therapy.
In a group of 5 women on long-term anticonvulsant and oral contraceptive therapy, the plasma ethynylestradiol (EE) concentration on 50 microgram EE daily was 11.1 +/- 4.5 pg/ml. These values were at the lower end of the range found in normal women in this laboratory taking 30 microgram EE daily (6-190 pg/ml). Four women have been studied prospectively for 3 months, over 1 cycle before and 2 cycles during phenobarbital 30 mg b.i.d. therapy. Significant falls in the plasma EE concentration were seen in two women (from 104.8 +/- 13.4 to 37.7 +/- 2.0 pg/ml and from 125.6 +/- 23.8 to 34.8 +/- 6.7 pg/ml p less than 0.01) and breakthrough bleeding was seen in both women. No changes in plasma concentrations of follicle stimulating hormone, progesterone, norethindrone or norgestrel were seen. There was a significant increase in the sex hormone binding globulin capacity from 100.7 +/- 5.8 to 133.3 +/- 1.2 nmoles/1 (p less than 0.05). These changes are consistent with the known microsomal enzyme inducing effect of phenobarbital.
D000293:Adolescent; D000328:Adult; D000927:Anticonvulsants / Q000494:pharmacology*; D003276:Contraceptives, Oral / Q000494:pharmacology*; D004347:Drug Interactions; D004997:Ethinyl Estradiol / Q000097:blood; D005260:Female; D012739:Gonadal Steroid Hormones / Q000097:blood; D006801:Humans; D009640:Norethindrone / Q000097:blood; D009644:Norgestrel / Q000097:blood; D010634:Phenobarbital / Q000494:pharmacology*; D011446:Prospective Studies; D012189:Retrospective Studies
D000927:Anticonvulsants; D003276:Contraceptives, Oral; D012739:Gonadal Steroid Hormones; D009644:Norgestrel; D004997:Ethinyl Estradiol; D009640:Norethindrone; D010634:Phenobarbital