Drug interactions are increasingly common, since the clinical practice is getting more complex with the flood of new drugs. Simultaneously, the increased life expectancy of the population increases the number of individuals likely to receive multiple prescriptions. Cytotoxic drugs generally have a narrow therapeutic index, and are delivered at doses close to toxic levels. Consequently, a slight increase of the biological activity caused by an interaction with other concomitantly administered drugs could be deleterious for the patient. Interactions between drugs can sometimes also be used in a positive way, i.e. to increase the therapeutic ratio. Interactions between different cancer treatment modalities have attracted considerable interest. However, much less interest has been denoted to interactions between anticancer drugs and other pharmaceuticals. The purpose of this review is to summarize data about the interactions between anticancer drugs and other clinically used drugs with regard to effects on tumor and toxicity.