- The use of potent P-gp inhibitors (including cyclosporine, erythromycin, ketoconazole, itraconazole, quinidine, Phenobarbital salt with quinidine, ritonavir, valspodar, verapamil) and potent P-gp inducers (including St John's wort, rifampicin) has to be avoided during treatment with afatinib. Any exemptions to this have to be discussed with the principal investigator.