CONTRAINDICATIONS AND DRUG INTERACTIONS
Some of the contraindications to the use of estrogens, progestins, and estrogen–progestin combinations are presented in Table 61.4.
Certain concomitantly administered drugs may in-terfere with the effectiveness of the oral contraceptives or lead to an increased incidence of breakthrough bleeding. These include rifampin, isoniazid, ampicillin, neomycin, penicillin V, chloramphenicol, sulfonamides, nitrofurantoin, phenytoin, barbiturates, primidone, analgesics, and phenothiazines.
The oral contraceptives also may decrease the effec-tiveness of anticoagulants, anticonvulsants, tricyclic an-tidepressants, guanethidine, and hypoglycemic agents. The causes of such drug interactions include alterations in hepatic microsomal drug-metabolizing enzymes, competition for binding sites on plasma proteins, and enhanced excretion.
Raloxifene absorption is inhibited by cholestyra-mine.