Reduced contraceptive efficacy w/ hepatic enzyme-inducers eg, carbamazepine, griseofulvin, modafinil, nelfinavir, nevirapine, oxcarbazepine, phenytoin, phenobarb, primidone, ritonavir, St. John's wort, topiramate, rifabutin, rifampicin & rufinamide. May result to contraceptive failure w/ aprepitant & bosentan. May increase incidence of side effects w/ amprenavir, sitaxentan, atorvastatin & tacrolimus. May enhance or reduce anticoagulant effect of coumarins & increase clearance of analgesics (eg, paracetamol & morphine). May antagonize hypoglycemic effect of antidiabetics & anticoagulant effect of phenindione. May increase plasma conc of ciclosporin, selegiline & tizanidine. May reduce plasma conc of lamotrigine.