This one has a real mechanism behind it (caffeine is a mild diuretic) and a real-world outcome that doesn’t match the alarm the mechanism implies.

A 2014 crossover trial published in PLOS ONE ^[1] directly compared habitual coffee drinkers’ hydration status when drinking coffee versus an equivalent volume of water, across a free-living (not lab-confined) study period. It found no significant difference in any hydration marker measured — body mass, urine osmolality, blood markers — between the coffee and water conditions.

A 2003 review in the Journal of Human Nutrition and Dietetics ^[2] covers why: caffeine does have a mild diuretic effect, but tolerance to that effect develops quickly in regular consumers, and at typical intake the fluid volume of the coffee itself more than offsets any extra fluid loss. The net effect at habitual, moderate intake is close to neutral for hydration status.

The diuretic mechanism is real. It’s just small enough, and offset enough, that it doesn’t show up as actual dehydration in the people who were tested drinking their normal amount of coffee.

Where this could genuinely shift: very high, atypical caffeine doses, or someone not habituated to caffeine at all, might see a more noticeable diuretic effect. For the ordinary “I have a few coffees a day” case this myth is aimed at, the controlled evidence doesn’t support “coffee dehydrates you” as a practical concern.