The testicles produce sperm that travels through a tube called the vas deferens. The vas deferens continues to travel through the prostate before joining with the vas on the other side to become the urethra. During a vasectomy, each vas deferens are cut and sealed off. This blocks sperm from reaching the semen ejaculated from the penis, creating semen without sperm. The testicles produce less sperm after a vasectomy, but they are absorbed by the body internally.
A vasectomy is more effective than any other method of birth control, besides abstinence. Because vasectomies don’t affect testosterone levels, the side effects of vasectomies are slim to none.