Decreased bioavailability w/ rifampicin, rifabutin, phenytoin; carbamazepine, phenobarb & isoniazid. Increased &/or prolonged effects including side effects w/ terfenadine, astemizole, mizolastine, cisapride, triazolam, oral midazolam, dofetilide, quinidine, pimozide, CYP3A4-metabolised HMG-CoA reductase inhibitors (eg, simvastatin & lovastatin). Negative inotropic effects w/ Ca-channel blockers. Reduce dose during co-administration w/ oral anticoagulants; HIV PIs eg, ritonavir, indinavir, saquinavir; certain antineoplastic (eg, vinca alkaloids, busulphan, docetaxel & trimetrexate) & immunosuppressive agents (eg, cyclosporin, tacrolimus, sirolimus); CYP3A4-metabolised Ca-channel blockers eg, dihydropyridines & verapamil.