Curcumin as a natural product has many biological and pharmacological properties. Curcumin has preventive and therapeutic effects on cancer cells through different cellular and molecular mechanisms. It has low side effects and it is possible to use it as a supplement in patients during cancer therapy. Several clinical studies were investigated for efficacy of curcumin in patients with cancer. Results showed that curcumin reduced side effects induced by anticancer therapy and radiotherapy in patients. In the present work, the clinical studies of curcumin were reviewed and several parameters such as dose of curcumin, placebo group, and size of sample and overall results of each study were discussed.