Drug Interaction Report

Consumer information for this interaction is not currently available.

ADDITIONAL CONTRACEPTION RECOMMENDED: Coadministration with inducers of CYP450 3A4 may decrease the plasma concentrations of estrogens and progestins. Estrogens have been shown in in vitro and in vivo studies to be partially metabolized by CYP450 3A4, and other steroids including progestins are also believed to undergo metabolism by this isoenzyme. The interaction has been reported primarily with oral contraceptives. There have been case reports of menstrual breakthrough bleeding or unwanted pregnancy in women receiving low-dose oral contraceptives following the addition of known CYP450 3A4 inducers such as carbamazepine, phenobarbital, phenytoin, rifampin, and St. John's wort. Inadequate response to estrogen replacement therapy has also been reported in a patient treated with phenytoin. Aminoglutethimide, a CYP450 3A4 inducer, has been shown to decrease medroxyprogesterone and megestrol serum levels by 74% in six patients stabilized on their progestin regimen.

MANAGEMENT: Pharmacologic response to estrogens and progestins should be monitored more closely whenever a CYP450 3A4 inducer is added to or withdrawn from therapy, and the hormone dosage adjusted as necessary. For patients receiving hormonal contraceptives, additional or alternative non-hormonal birth control may be advisable during concomitant therapy with CYP450 3A4 inducers. Additional or alternative non-hormonal birth control may be recommended beyond discontinuation of the CYP450 3A4 inducer(s). Individual product labeling should be consulted for specific time frames. Intrauterine systems are unlikely to be significantly affected because of their local action. Input from a gynecologist or similar expert on adequate contraception, including emergency contraception, should be sought as needed. Patients using replacement therapy should be advised to notify their physician if they experience inadequate control of symptoms associated with estrogen deficiency (e.g., nocturnal sweating, vasomotor disturbances, atrophic vaginitis) or changes in the uterine bleeding profile.