Altered desogestrel blood levels, reduced effectiveness, or unexpected bleeding w/ primidone, phenytoin, carbamazepine, oxcarbazepine, felbamate, topiramate, phenobarb; rifampicin, rifabutin; ritonavir, nelfinavir, nevirapine, efavirenz; boceprevir, telaprevir; griseofulvin; bosentan; St. John's wort; clarithromycin, erythromycin; ketoconazole, itraconazole, fluconazole; diltiazem. Increased effect of cyclosporine-containing medicines. Decreased effect of lamotrigine.