As previously mentioned, the caffeine in coffee is a diuretic, which means it makes you pee more. How does it do this, you ask? Dr. Quayle explains: “Diuretics are a class of drugs that increase the flow of urine…by removing sodium and chloride from the body in the urine. The sodium and chloride, in turn, draw excess water from the body [and] this presents as an increase in urine volume.” In other words, caffeine inhibits sodium reabsorption in the body and the result is more pee.
The diuretic action of caffeine described above is the main reason that coffee will send you to the bathroom more than usual, but it’s not the only reason. Per Dr. Quayle, the stimulant properties of caffeine are also thought to irritate the bladder, causing it to contract—and, since the bladder is already full from the diuretic effect, these contractions create a sense of urgency (i.e., you’ve gotta pee right away).