Vasectomies are not immediately effective. You will need to wait until a doctor confirms there is no sperm in your semen to stop using other forms of birth control. This can take up to 3 months in some cases, and you may need to ejaculate up to 20 times before you can be sure. In rare cases, a vasectomy can fail. Vasectomies are an effective method of birth control but will not protect against sexually transmitted infections such as HIV. A vasectomy cannot grow back, and if effective can only be reversed through another surgical procedure.