Drinking coffee will dehydrate you.
MYTHBUSTER Nope. While it’s true that caffeine is a mild diuretic, which means the stimulant will cause your kidneys to expel more sodium and water than they would if you were sipping water, coffee is still a liquid, so its hydrating properties balance out any diuretic effects, says Vince Bufalino, M.D., senior medical director at Advocate Health
Care in Naperville, IL.
In a study published in PLOS One, researchers looked at the blood and urine of participants who drank either coffee or an equivalent amount of water over a period of three days and found no significant difference in hydration levels.