Why Does Coffee Make You Poop?
For a sizable share of coffee drinkers, the morning cup does double duty: it wakes up the brain and then sends them to the bathroom. This is one of the most reliably reproduced effects in gut physiology, and it works faster than almost anything else you can drink.
A measurable rush in the colon
In a classic 1990 study at the Royal Hallamshire Hospital in Sheffield, researchers first surveyed 99 healthy young volunteers: 29 percent said coffee gave them the urge to defecate, and most of those were women. The team then measured the lower colon directly with pressure sensors in 14 volunteers. In the people who normally felt the effect, rectosigmoid motility rose within four minutes of drinking black coffee and stayed elevated for at least 30 minutes. Hot water did nothing.
Four minutes is far too fast for coffee to have travelled through the stomach and small intestine to reach the colon. The speed tells us the trigger is a reflex: coffee in the stomach signals the colon downstream, likely through hormones and nerve pathways that link the two ends of the gut.
It is not just the caffeine
The surprise from the Sheffield experiment was that decaffeinated coffee produced the same response in the same people. Caffeine clearly plays a role: a 2023 study in children found caffeinated coffee stimulated the colon more than decaf, confirming caffeine as a genuine colonic stimulant. But something else in the cup is also at work. A 2022 laboratory study found that coffee contracts gut smooth muscle even without caffeine, through a mechanism involving muscarinic receptors, and coffee contains roughly a thousand biologically active compounds, including chlorogenic acids, that are candidates for the effect.
The gastrocolic reflex adds another layer. Eating or drinking after a period of fasting naturally tells the colon to make room, and a morning coffee arrives at exactly the moment this reflex is strongest. The drink both rides the reflex and amplifies it.
Why only some people feel it
In the Sheffield study the effect was real but selective: non-responders showed no colon response at all to the same drink. Whether the difference comes from gut sensitivity, habit, or anatomy is still an open question, and it is why coffee is a reliable laxative for some people and just a beverage for everyone else.
Facts checked: October 24, 2026
Sources: Brown, S. R., Cann, P. A., and Read, N. W. (1990). Effect of coffee on distal colon function. Gut, 31(4):450. PMC1378422. Mehta, V. et al. (2023). Effect of Caffeine on Colonic Manometry in Children. Journal of Pediatric Gastroenterology and Nutrition. PMC9760462. Hegde, S. et al. (2022). Mechanistic Study of Coffee Effects on Gut Microbiota and Motility in Rats. Nutrients. PMC9699609.