Dietary supplements (DS) may influence cancer prognosis. Their use in cancer patients has been described in the United States, but data are largely lacking in Europe and notably in France. The present study's objectives were (1) to assess DS use and its sociodemographic, lifestyle, and dietary correlates in a large sample of French cancer survivors; (2) to evaluate the involvement of physicians in such DS use; and (3) to assess the extent of potentially harmful practices. Data were collected by self-administered web-based questionnaires among participants of the NutriNet-Sante cohort. Data on DS use was available for 1081 cancer survivors. DS users were compared to nonusers with unconditional logistic regressions. DS use was reported by 62% of women and 29% of men. Vitamins D, B-6, C and Mg were the most frequently consumed nutrients. 14% of cancer survivors initiated DS use after diagnosis. For 35% of the DS consumed, subjects did not inform their attending physician. DS use was associated with a healthier lifestyle (normal weight, never smoking and better diet) and substantially contributed to nutrient intake. 18% of DS users had potentially harmful DS use practices, such as the simultaneous use of vitamin E and anticoagulant/antiplatelet agents, the use of beta-carotene and smoking or the use of phyto-oestrogens in hormone-dependent cancer patients. The present study suggests that DS use is widespread among cancer survivors, a large amount of that use is performed without any medical supervision and a substantial proportion of that use involves potentially harmful practices. Physicians should be encouraged to more routinely discuss DS use with their cancer patients.