The claim is everywhere: coffee dries you out. Studies suggest the truth is milder.
Caffeine does make you pass more urine. The effect is small at normal doses. It also fades in people who drink coffee every day.
What studies found
Trials that compared coffee with water found similar hydration after several hours. In regular drinkers, a moderate amount of coffee counted toward daily fluid intake.
Very large single doses can have a stronger effect. That is a reason to avoid extreme amounts, not a reason to skip your morning cup.
The takeaway
A cup of coffee is more than 98 percent water. Count it. If you do a hard workout or sit in the heat, drink plain water too.
