Bacillus Calmette-Guérin (BCG) is a vaccine used to treat bladder cancer and is administered directly into the bladder through the urethra. It is a weakened form of the tuberculosis vaccine that stimulates the immune system to target and destroy cancer cells. BCG is a form of immunotherapy, with localized anticancer effects and primarily local side effects. It is often used in conjunction with other treatments as part of a comprehensive strategy to eradicate cancer tumours in the bladder.