Doxofylline (Doxofree) should not be administered together with other xanthine derivatives. Toxic synergism with ephedrine has been demonstrated for xanthine. Like other xanthines, concomitant therapy with erythromycin, troleandomycin, lincomycin, clindamycin, allopurinol, cimetidine, ranitidine, propranolol and anti-flu vaccine may decrease the hepatic clearance of xanthines, causing an increase in blood levels. No evidence of a relationship between Doxofylline (Doxofree) serum concentration and toxic events has been reported.