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As the oestrogen in Vagifem is administered within the vagina and due to the low levels of estradiol released, it is unlikely that any clinically relevant drug interactions will occur with Vagifem. However, the metabolism of oestrogens may be increased by concomitant use of substances known to induce drug-metabolising enzymes, specifically cytochrome P450 enzymes, such as anticonvulsants (e.g. phenobarbital, phenytoin, carbamazepine) and anti-infectives (e.g. 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 oestrogens.
