A widely used laxative creates profound changes in the gut microbiome, according to experiments in mice.

Polyethylene glycol — a component of many over-the-counter laxatives — triggers mild diarrhoea by drawing water into the colon. To determine how such ‘osmotic’ laxatives affect gut bacteria, Justin Sonnenburg and his colleagues at the Stanford University School of Medicine in California gave polyethylene glycol to mice for six days. The laxatives triggered an immune response and short-term changes to the rodents’ gut cells. The treatment also drove a common family of gut bacteria called S24-7 to extinction and allowed other, less-prevalent species to flourish.

Two weeks after the mice stopped taking the laxatives, their gut bacteria were less diverse than before the course of drugs. In humans, a reduction in diversity of the gut microbiome has been linked to bowel disorders such as Crohn’s disease.