Cigarette smoking, practiced by over a billion people worldwide, is considered a leading cause of disease, accounting for over six million deaths each year. Many people don’t quit smoking, despite expressing a desire to do so, because they are concerned about the substantial weight gain that often accompanies smoking cessation. The cause of this weight gain is unclear, as studies suggest that most people don’t eat more after quitting smoking. In a study published today in Nature, Weizmann Institute of Science researchers report discovering that obesity developing after “smoking cessation” in mice may be driven by the weight-modulating compounds released by their gut microbes.