Caffeine inhibits the secretion of the antidiuretic hormone (ADH), so it increases diuresis, but this effect is small and transient. <
Caffeine in amounts up to 400 mg/day (~4 cups of coffee) does not results in net dehydration, since the amount of water lost due to slightly increased diuresis is much smaller than the amount of water in coffee. <
In some people, caffeine may irritate the bladder and thus trigger more frequent urination, but this is not increased diuresis and the total volume of the urine is not increased.