DRUG INTERACTIONS 9. Drug Interactions
Interactions between ethinyl estradiol and other substances may lead to decreased or increased serum ethinyl estradiol concentrations.
Decreased ethinyl estradiol plasma concentrations may cause an increased incidence of breakthrough bleeding and menstrual irregularities and may possibly reduce efficacy of the combination oral contraceptive.
Reduced ethinyl estradiol concentrations have been associated with concomitant use of substances that induce hepatic microsomal enzymes, such as rifampin, rifabutin, barbiturates, phenylbutazone, phenytoin sodium, griseofulvin, topiramate, some protease inhibitors, modafinil, and possibly St. John’s wort.
Substances that may decrease plasma ethinyl estradiol concentrations by other mechanisms include any substance that reduces gut transit time and certain antibiotics (e.g. ampicillin and other penicillins, tetracyclines) by a decrease of enterohepatic circulation of estrogens. During concomitant use of ethinyl estradiol containing products and substances that may lead to decreased plasma steroid hormone concentrations, it is recommended that a nonhormonal back-up method of birth control be used in addition to the regular intake of Trivora (levonorgestrel and ethinyl estradiol tablets-triphasic regimen). If the use of a substance which leads to decreased ethinyl estradiol plasma concentrations is required for a prolonged period of time, combination oral contraceptives should not be considered the primary contraceptive.
After discontinuation of substances that may lead to deceased ethinyl estradiol plasma concentrations, use of a nonhormonal back-up method of birth control is recommended for 7 days. Longer use of a back-up method is advisable after discontinuation of substances that have led to induction of hepatic microsomal enzymes, resulting in decreased ethinyl estradiol concentrations. It may take several weeks until enzyme induction has completely subsided, depending on dosage, duration of use, and rate of elimination of the inducing substance.
Some substances may increase plasma ethinyl estradiol concentrations. These include:
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Competitive inhibitors for sulfation of ethinyl estradiol in the gastrointestinal wall, such as ascorbic acid (vitamin C) and acetaminophen.
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Substances that inhibit cytochrome P450 3A4 isoenzymes such as indinavir, fluconazole, and troleandomycin. Troleandomycin may increase the risk of intrahepatic cholestasis during coadministration with combination oral contraceptives.
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Atorvastatin (unknown mechanism).
Ethinyl estradiol may interfere with the mechanism of other drugs by inhibiting hepatic microsomal enzymes or by inducing hepatic drug conjugation, particularly glucuronidation. Accordingly, tissue concentrations may be either increased (e.g. cyclosporine, theophylline, corticosteroids) or decreased.
The prescribing information of concomitant medications should be consulted to identify potential interactions.
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