As patients live longer after receiving an organ transplant, there is a need to reduce the long-term side effects of the drugs used to prevent organ rejection. In particular, long-term use of these drugs increases the risk of skin cancer. Skin cancer is now a leading cause of illness and disfigurement after kidney, liver, heart, and lung transplantation. Given the increased risk and burden of skin cancer in transplant recipients, prevention is critical. Nicotinamide is a form of Vitamin B3 that has been shown to protect against skin cancer in the general population. However, it is unclear whether nicotinamide is effective among immune-suppressed transplant recipients. Investigators will conduct a clinical trial involving multiple transplant centres in Canada to evaluate whether oral nicotinamide (500 mg twice daily) is effective and safe for preventing skin cancer. Investigators will recruit 396 high-risk adult kidney, liver, heart, and lung transplant patients who have previously had at least one skin cancer. Patients will receive nicotinamide or sham tablets for up to 4 years. The results will inform efforts to improve the long-term health of transplant recipients.