Mebendazole is an antiparasitic drug with over 40 years of safe use. Recently mebendazole was repurposed for glioblastoma therapy. Three polymorphs of mebendazole exist, but the relative polymorph content for existing drugs varies, and the therapeutic anti-cancer relevance of the different polymorphs was unknown. As an oral drug mebendazole polymorph C is a superior form, and it reaches the brain and brain tumors in effective concentrations. Efficacy is further improved by combining mebendazole with a P-glycoprotein inhibitor. Mebendazole may also be used for therapy of other cancers, as well as a chemo-preventative agent.