This review article goes through many researches based on the effectiveness of the neurotransmitter serotonin on cancer cells and also the impact of SSRIs (selective serotonin reuptake inhibitors) drugs on the cause of certain types of cancers. Serotonin has been shown to be a mutagenic factor for a wide range of normal and tumor cells. Serotonin exhibits a growth stimulatory effect in aggressive cancers and carcinoids usually through 5- HT1 and 5-HT2 receptors. In contrast, low doses of serotonin can inhibit tumor growth via the decrease of blood supply to the tumor, suggesting that the role of serotonin on tumor growth is concentration-dependent. Serum serotonin level was found to be suitable for prognosis evaluation of urothelial carcinoma in the urinary bladder, adenocarcinoma of the prostate and renal cell carcinoma.The mechanism that connect serotonin with tumor evolution seem to be related to the serotonin impact on tumor associated macrophages.