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The metabolism of estrogens (and progestagens) may be increased by concomitant use of substances, known to induce drug-metabolising enzymes, specifically cytochrome P-450 enzymes eg, anticonvulsants (eg, phenobarbital, phenytoin, carbamazepine) and anti-infectives (eg, rifampicin, rifabutin, nevirapine, efavirenz).
Ritonavir and nelfinavir, although known as strong inhibitors, by contrast exhibit inducing properties when used concomitantly with steroid hormones.
Herbal preparations, containing St. John's wort (Hypericum perforatum), may induce the metabolism of estrogens.
At transdermal administration, the first-pass effect in the liver is avoided and thus transdermally applied estrogens (and progestagens) might be less affected than oral hormones by enzyme inducers.
Clinically, an increased metabolism of estrogens and progestagens may lead to decreased effect and changes in the uterine bleeding profile.
Incompatibilities: Not applicable.
