Interactions of drugs with oral contraceptives

Simultaneous administration of oral contraceptives and antihypertensive anticoagulant or diabetic drugs can reduce the effectiveness of the latter can cause development of imipramine resistance and can increase the blood concentration and possibly the action of total thyroxin and total and free cortisol. Oral contraceptives can raise blood pressure. Estrogen promotes sodium retention in the proximal renal tubule; progestagens intensify this effect. Risk of thromboembolism is enhanced by taking oral contraceptives by increasing the number of coagulation factors. Steroid contraceptives endanger diabetics by reducing glucose tolerance due to formation of a so-called insulin block. On the other hand some substances can enhance or reduce the effectiveness of oral contraceptives. Vitamin C in amounts of 1g/day has been found to intensify the action of contraceptives by enhancing systemic estrogen action due to resorption of larger unmetabolized amounts of ethinyl estradiol. The tuberculostatic drug rifampicin is an enzyme reducing substance which accelerates breakdown of estrogen and progestagen components for oral contraceptives. Plasma elimination time of these steroids is cut in half and elimination in the urine quadrupled. Anticonvulsive drugs such as phenobarbitol reduce contraceptive effectiveness by enzyme induction in the liver and also increase the capacity of sex hormone binding globulin. Pregnancies while taking contraceptives have been reported in women taking phenobarbitol phenytoin and primidone or combinations thereof. Antibiotics or sulfonamides may reduce effectiveness of contraceptives by slowing reabsorption of estrogens due to reduced intestinal microflora although this has not been definitively demonstrated.