NST research

Does caffeine dehydrate you?

Not meaningfully. Caffeine is a mild diuretic only at large doses and mainly in people not used to it, and the roughly 100 mL of extra urine is far less than the fluid in the drink. Regular drinkers are tolerant, exercise cancels the effect, and caffeinated drinks count towards your daily fluid.
Research-basedReviewed by Gabriel SilvaLast verified 12 September 2026Next review due 14 March 2027Report a correction

The evidence-led answer

NST verdict

No, not in any way that matters for staying hydrated. Caffeine has a mild diuretic effect, but only at fairly large doses, about two to three cups of coffee in one go, and mainly in people who have not had caffeine for days or weeks. Even then the extra urine is small, around 100 mL on average in the research, which is far less than the fluid you just drank, so a cup of coffee or tea still leaves you with more fluid than you started with. Regular coffee and tea drinkers build up a strong tolerance and show no measurable diuretic effect from normal servings. Exercise cancels the effect entirely. Caffeinated drinks count towards your daily fluid, and you do not need to "drink a glass of water for every coffee".

Is caffeine a diuretic?

Mildly, and mostly under specific conditions. A large single dose, about 250 to 300 mg or more, roughly two to three cups of coffee, briefly increases urine output in people who have been caffeine free for days or weeks. In a pooled analysis of 16 studies, the effect at rest was real but modest. Ordinary servings, in people who drink caffeine regularly, showed essentially no diuretic effect at all.

Does that actually dehydrate you?

No. The pooled analysis put the average extra urine at about 109 mL, around 16% more than without caffeine. A mug of coffee is roughly 250 mL of fluid, so even in the scenario where caffeine does increase urine output, you still come out well ahead. And when researchers gave 50 men four cups of coffee a day for three days and compared it with water, there was no difference in total body water, 24 hour urine volume, or any blood or urine marker of hydration. Coffee hydrated them just as well as water.

What if you drink coffee every day?

Then the diuretic effect essentially disappears. A strong tolerance develops within a few days of regular caffeine intake, and habitual tea and coffee drinkers show no meaningful diuretic response to normal servings. The three day study above used men who habitually drank three to six cups a day, which is part of why coffee and water came out identical.

What about caffeine before a workout?

Exercise abolishes the effect. In the pooled analysis, caffeine taken before or during exercise produced no significant increase in urine output, compared with a small but significant effect at rest. So a pre workout coffee or caffeine supplement is not a hydration problem. One nuance: women showed a larger resting diuretic response than men in that analysis, though it was still modest and still did not amount to net fluid loss.

What this means in practice

Count your coffee, tea and other caffeinated drinks towards your daily fluid, because that is what the evidence says they are. You do not need to match every coffee with a glass of water. If you are new to caffeine, or you take a very large dose, you might notice you pee a bit more for an hour or two; that is a mild, temporary effect, not dehydration. In hot weather or during hard exercise, what keeps you hydrated is replacing sweat losses and drinking to thirst. Whether the fluid had caffeine in it is not the issue.

Practical meaning

Count coffee, tea and other caffeinated drinks towards your daily fluid, since that is what the evidence says they are. You do not need a glass of water per coffee. If you are new to caffeine or take a very large dose you may pee a little more for an hour or two, which is not dehydration. In heat or hard exercise, hydration is about replacing sweat losses and drinking to thirst, not about whether the fluid had caffeine in it.

Limitations and uncertainty

The three day trial used 50 habitual male coffee drinkers at a moderate dose, so it shows coffee hydrates like water for regular drinkers, not for first time users, women specifically, or very high doses. The review's search ended in 2002; the 2014 pooled analysis agrees. Women were more susceptible to the resting diuretic effect than men, though it was still modest with no sign of net dehydration. This covers healthy adults and ordinary intakes only.

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Evidence behind this page

What the evidence says, and where it stops.

Caffeine is a mild diuretic at large acute doses in people not used to it, and the effect is small

moderate confidence

Mildly, and mostly under specific conditions. A large single dose (about 250 to 300 mg or more, roughly 2 to 3 cups of coffee) briefly raises urine output in people who have been caffeine free for days or weeks; a pooled analysis of 16 studies put the resting effect at 0.54 and the overall effect at 0.29, corresponding to about 109 mL (16%) more urine. That is far less than the fluid volume of the drink itself, so a caffeinated drink is still a net fluid gain. Ordinary servings in habitual consumers showed essentially no diuretic effect.

Study context and sources
Who it may apply to
Healthy adults in acute caffeine dosing studies (median 300 mg); review covers caffeine deprived and habitual consumers.
Limitations
Acute dose studies, not multi day balance; heterogeneity across protocols; the review's search ended in 2002.

Sources

Caffeinated drinks do not cause net dehydration; habitual drinkers are tolerant

moderate confidence

No. When 50 habitual male coffee drinkers consumed four cups of coffee a day for three days and were compared with water, there was no difference in total body water, 24 hour urine volume, or any blood or urine marker of hydration, coffee hydrated them as well as water. A strong tolerance to the diuretic effect develops within days of regular use, so in habitual tea and coffee drinkers normal servings have no meaningful diuretic action. The average extra urine seen in the pooled analysis (about 109 mL) is much smaller than the fluid volume of the drink, so even when caffeine does raise urine output the drink is still a net gain.

Study context and sources
Who it may apply to
Habitual male coffee drinkers (3 day trial); review covers habitual consumers generally.
Limitations
The trial is 50 habituated men at a moderate dose, not first time users, women specifically, or very high doses.

Sources

Exercise abolishes the diuretic effect; caffeinated drinks count towards daily fluid

moderate confidence

Exercise cancels the effect. In the pooled analysis, caffeine taken before or during exercise produced no significant increase in urine output (effect 0.10, not significant) versus a small significant effect at rest, so a pre workout coffee or caffeine supplement is not a hydration problem. Women showed a larger resting diuretic response than men (0.75 vs 0.13) but it was still modest and still did not amount to net fluid loss. In practice, caffeinated drinks count towards daily fluid intake; there is no need to add a glass of water per coffee; in heat or hard exercise, hydration is driven by replacing sweat losses and drinking to thirst, not by whether the fluid contained caffeine.

Study context and sources
Who it may apply to
Healthy adults; pooled analysis exercise and sex subgroups; practical guidance is general.
Limitations
Subgroup analyses within a modest pooled analysis; the 'fine before exercise' point should not be stretched to extreme heat or very high doses without the thirst and sweat replacement caveat.

Sources

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