Endoxifen is an effective metabolite of tamoxifen, a commonly used chemotherapy drug, and has been detected at the final effluent of a municipal wastewater treatment plant (WWTP). Its presence in the environment could bring negative effects to aquatic lives. This research investigated the use of ultraviolet radiation (253.7 nm) and natural sunlight to photodegrade endoxifen in water and wastewater, the generation of photodegradation by-products (PBPs) and their toxicity. Endoxifen concentration in water was reduced by 99.1% after 35 seconds of UV light generating two toxic PBPs. Experiments in wastewater at UV light doses used for disinfection resulted in reduction of endoxifen by 30 to 71%. Endoxifen concentration in wastewater was reduced by at least 83% after 150 minutes of solar radiation generating eight PBPs. Seven of these PBPs are potentially more toxic than endoxifen itself. Therefore, highly toxic PBPs are potentially generated at WWTPs if endoxifen is present in wastewater.