(1) Background: Little data are available in Western countries regarding self-medication practices in the context of cancer. The aim of this study was to describe the prevalence of self-medication practices during (cancer patients) and after cancer (cancer survivors). (2) Methods: This multicenter, cross-sectional, and online study was designed to assess self-medication prevalence. Other objectives were explored, notably the medication types, the perceived risks, and the relation with symptoms and quality of life. (3) Results: Among the 518 patients analyzed, 56.4% declared they practiced self-medication. Dietary supplements and pain medications were used by more than half of the patients. Self-medication was practiced in order to manage the adverse effects of anticancer therapies (63.8%), for which pain was the leading indication (39%), and to improve the efficacy of anticancer therapies (43.8%, cancer patients). Patients believed that self-medication could not lead to drug interactions with anticancer therapies (84.9%, cancer patients), or to adverse effects (84.6%, cancer patients and survivors). Self-medication practices were associated with altered social functioning, pain, insomnia, and financial difficulties. (4) Conclusions: Self-medication was performed by more than half of the responders (ongoing or past cancer) and could be a marker of the undermanagement of cancer and treatment-related adverse effects.