Smoking can gradually and permanently damage blood vessels throughout the body, including those that carry blood to the penis. This can make it difficult to get or maintain an erection (impotence). Quitting smoking may help prevent new damage from occurring inside the blood vessels.
Smoking can also affect a man's ability to get or maintain an erection in other ways that are not well understood. Nicotine may make the blood vessels narrower for a short time, making it more difficult to get enough blood into the penis for a normal erection. Men who quit smoking often have fewer problems achieving a normal erection.
Women who smoke take longer to become pregnant. However, women who quit smoking before they try to become pregnant are as likely to become pregnant as women who have never smoked.
WebMD Medical Reference from Healthwise