Effect of oral contraceptives on plasma clearance

The effects of chronic steroid contraceptive therapy on drug clearance from plasma were studied by using plasma antipyrine, phenylbutazone, and cholecalciferol half-lives in women. After 3 mo of oral steroid therapy (Norinyl, 2 mg; norethindrone + mestranol), the antipyrine half-life was increased in 3 of 6 subjects, phenylbutazone half-life was not consistently altered, and vitamin D₃ half-life was increased in 3 of 4 patients. After 1 to 7 yr of oral steroid therapy, the antipyrine half-life was longer while taking the contraceptive than when the contraceptive treatment was discontinued in 4 of 6 subjects, whereas that of phenylbutazone was not consistently altered.