There is disclosed a multidrug resistance inhibitor for overcoming a multidrug resistance of cancer, or an agent for enhancing the activity of anti-cancer agents, which comprises as an active ingredient a compound of formula (I) or a pharmaceutically acceptable salt thereof: ##STR1## wherein n is an integer of 5 to 12, R.sub.1 and R.sub.2 are each independently benzyl, of which a phenyl ring may be substituted by 1 to 5 substituents selected from hydroxy, (C.sub.1 -C.sub.4)alkyl, (C.sub.1 -C.sub.4)alkoxy and halogen, and/or by methylenedioxy, with the proviso that R.sub.1 and R.sub.2 are simultaneously not a compound of formula (II) ##STR2##
