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.
PIP: In a group of 5 women on long-term anticonvulsant and (OC) oral contraceptive therapy, the plasma (EE) ethinyl estradiol concentration on 50 mcg EE daily was 11.1 + or - 4.5 pg/ml. These values were at the lower end of the range found in normal women in this laboratory taking 30 mcg EE daily (6-190 pg/ml). 4 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 2 women (from 104.8 + or - 13.4 to 37.7 + or - 2.0 pg/ml and from 125.6 + or - 23.8 to 34.8 + or - 6.7 pg/ml, (P 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 + or - 5.8 to 133.3 + or - 1.2 nmoles/l (P 0.05). These changes are consistent with the known microsomal enzyme-inducing effect of phenobarbital.