Why drinking dehydrates you

Alcohol increases urine output partly by interfering with antidiuretic hormone, also called vasopressin or ADH. ADH normally instructs the kidneys to reabsorb water from the urine rather than excreting it. When ADH is suppressed by alcohol, the kidneys allow water to pass into the urine rather than returning it to circulation. The result is that you produce far more urine than the volume of fluid you consumed, creating a net negative fluid balance that progresses throughout the evening and into the early hours. This effect is strongest in the first few hours after alcohol consumption when blood alcohol levels are rising and ADH suppression is maximal. As blood alcohol falls, ADH is gradually restored, but the fluid deficit accumulated during the peak suppression period persists into the morning.

What is happening

The fluid loss from alcohol-induced diuresis is not pure water. Electrolytes, particularly sodium and potassium, are lost alongside water in the increased urine volume. This electrolyte loss is one reason that the hangover headache does not fully resolve with plain water intake. Plain water replaces fluid volume but does not restore the electrolyte balance, meaning the osmotic environment in tissues, including brain tissue, remains partially disrupted even after total fluid volume is normalised. The residual headache after drinking adequate water in the morning often reflects this electrolyte component rather than ongoing dehydration per se. Electrolyte-containing solutions, or a combination of water with food containing sodium and potassium, resolve the remaining symptoms more effectively than water alone.

Acetaldehyde, the primary metabolic product of alcohol breakdown in the liver, is far more toxic than alcohol itself and contributes substantially to the nausea, sweating, increased heart rate and general misery of a hangover independent of dehydration. Acetaldehyde is cleared by the enzyme aldehyde dehydrogenase, which requires NAD+ and adequate liver function to operate. Supporting acetaldehyde clearance through adequate B vitamins, particularly thiamine and B12 which support the enzymatic pathway, and adequate time is the primary mechanism of hangover resolution. No supplement or food dramatically accelerates this clearance. Time and sleep, during which liver metabolism continues at a steady rate, are the primary drivers of acetaldehyde resolution.

What to do next

The practical hangover management approach that best fits the actual mechanisms is electrolyte-enhanced water rather than plain water in the morning, food with sodium, potassium and B vitamins alongside it, and sufficient sleep to allow acetaldehyde clearance to complete. Greasy food, the cultural hangover supports, has no specific evidence base but provides caloric support, slows gut motility, and contains salt, which addresses the electrolyte component. Coffee may temporarily ease a headache through caffeine’s vascular and alertness effects, but it does not address the underlying fluid or alcohol-metabolism load. The expectation of rapid full recovery from a significant hangover is at odds with the biochemistry. The acetaldehyde metabolism takes the time it takes, regardless of what interventions are applied on top.

Sources: WHO alcohol guidance, NHS alcohol advice, NCBI sleep stages, WHO stress guidance.

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