No interaction studies have been performed.
The efficacy of oestrogens and progestogens might be impaired: The metabolism of oestrogens and progestogens may be increased by concomitant use of substances known to induce drug-metabolising enzymes, specifically the P450 enzymes 2B6, 3A4, 3A5, 3A7, such as anticonvulsants (e.g. phenobarbital, carbamazepine, phenytoin) and anti-infectives (e.g. rifampicin, rifabutin, nevirapine, efavirenz).
Ritonavir and nelfinavir, although known as strong inhibitors of CYP450 3A4, A5, A7, 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 oestrogens and progestogens via the CYP450 3A4 pathway.
Clinically, an increased metabolism of oestrogens and progestogens may lead to decreased effect and changes in the uterine bleeding profile.
Oestrogens might interfere with the metabolism of other drugs: Oestrogens per se may inhibit CYP450 drug-metabolising enzymes via competitive inhibition.
This is in particular to be considered for substrates with a narrow therapeutic index, such as: tacrolimus and cyclosporine A (CYP450 3A4, 3A3); fentanyl (CYP450 3A4); theophylline (CYP450 1A2).
Clinically this may lead to an increased plasma level of the affected substances up to toxic concentrations. Thus, careful drug monitoring for an extended period of time might be indicated and a dosage decrease of tacrolimus, fentanyl, cyclosporin A and theophylline may be necessary.