Does caffeine dehydrate you? What the coffee studies found

Key takeaways

• In a crossover trial, 50 male regular coffee drinkers were as hydrated on four cups of coffee a day (4 × 200 ml) as on the same volume of water.
• A 2003 review found that the caffeine in an everyday cup of tea or coffee, or a can of soft drink, does not seem to act as a diuretic.
• Larger doses, about 250–300 mg or more at once, can briefly raise urine output in people who have gone days or weeks without caffeine; regular drinkers become largely tolerant to this.
• Dehydration still appears on lists of effects of too much caffeine, so the myth is wrong at moderate intake rather than at every intake.

What the 2014 coffee hydration trial measured

The most direct test of the coffee-dehydration idea is a 2014 trial in 50 healthy men who already drank three to six cups of coffee a day. Each man spent three days drinking four 200 ml mugs of instant coffee a day and another three days drinking the same amount of water, in a counterbalanced order, so each was compared with himself. The coffee supplied 4 mg of caffeine per kg of body weight a day, which works out to 280 mg for a 70 kg man.

The researchers found no evidence of dehydration: in moderation, coffee hydrated these habitual drinkers about as well as water. Keep the limits in mind. Everyone in the trial was male and used to caffeine, the amount was moderate, and one of the authors was employed by PepsiCo, which the paper declared.

Is coffee a diuretic? It depends on the caffeine dose

Caffeine is often called a diuretic, something that makes you pass more urine. A 2003 review of caffeine and fluid balance found that this is true only in part. At the amounts in an everyday cup of tea or coffee or a can of cola, caffeine did not appear to increase urine output.

Larger doses are different. In the same review, a single dose of at least about 250 to 300 mg, which the authors equate to two or three cups of coffee, briefly increased urine output in people who had gone without caffeine for days or weeks. People who drink tea or coffee regularly become largely tolerant to this effect. For scale, the FDA lists 113 to 247 mg for a 12-ounce brewed coffee, so the effect is most likely when someone who has been off caffeine has a large dose in one go. Our brewed coffee page shows how much cups vary.

So the honest summary is not that caffeine has no diuretic effect. It is that moderate amounts, in people who drink caffeine regularly, do not appear to leave them short of fluid.

Why dehydration still appears on caffeine side-effect lists

If moderate coffee does not dehydrate you, why do health pages still mention it? MedlinePlus lists dehydration among the effects of too much caffeine, alongside restlessness, a fast heart rate, insomnia and anxiety. That fits the review: large amounts, especially when you are not used to them, can raise urine output for a while.

The two messages describe different doses. A couple of everyday coffees is not the situation those lists are warning about. If you regularly drink enough caffeine to get side effects like a racing heart or shakiness, dehydration is the least of the reasons to cut back; see how much caffeine is too much.

Does coffee count toward your daily fluid intake?

For regular coffee drinkers at moderate intake, the 2014 trial suggests it can: four cups a day hydrated the men about as well as the same volume of water. The review points the same way for standard servings of tea and soft drinks.

What the evidence does not cover is just as important. The trial included only men who were used to caffeine, and neither source looks specifically at energy drinks, so we cannot extend the finding to everyone or to every drink. Water is still the simplest drink for staying hydrated, and the stronger reason to watch your coffee is its effect on sleep, covered in caffeine and sleep.

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