What actually causes hangovers, and what helps?

Dehydration, acetaldehyde buildup, inflammation, poor sleep quality, and low blood sugar. Only time truly cures one. Water, food, and rest help; "hair of the dog" delays recovery. Paracetamol/acetaminophen is risky with alcohol in the liver — ibuprofen with food is generally safer.

Congeners — compounds in darker drinks like whisky, rum and red wine — worsen hangovers compared with clear spirits at the same alcohol dose.

Eating before and while drinking slows absorption, and alternating alcoholic drinks with water reduces both total intake and dehydration. Neither prevents a hangover from a heavy night.