Increased clearance which may lead to breakthrough bleeding &/or contraceptive failure w/ drugs that induce microsomal enzymes eg, phenytoin, barbiturates, primidone, carbamazepine, rifampicin, possibly oxcarbazepine, topiramate, felbamate, griseofulvin, St. John's wort. Decreased steady state conc & systemic exposures w/ rifampicin (CYP3A4 inducer). Co-administration w/ HIV/HCV PIs & NNRTIs can increase or decrease plasma conc of estrogen or progestin. Strong & moderate CYP3A4 inhibitors eg, azole antifungals (eg, itraconazole, voriconazole, fluconazole), verapamil, macrolides (eg, clarithromycin, erythromycin), diltiazem & grapefruit juice can increase plasma conc of estrogen or progestin or both. May affect metabolism of other drugs; plasma & tissue conc may either increase (eg, ciclosporin) or decrease (eg, lamotrigine). May influence results of certain lab tests including biochemical parameters of liver, thyroid, adrenal & renal function, plasma levels of (carrier) proteins eg, corticosteroids binding globulin & lipid/lipoprotein fractions, carbohydrate metabolism parameters & parameters of coagulation & fibrinolysis.