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‘Tis the season for holiday parties—which may include a flurry of festive cocktails that sound too delicious not to try. Overindulge and you may wake up feeling groggy with a pounding head and a mouth drier than a month-old pine tree. Hangovers are no fun, that’s for sure. So how did you go from being merry to feeling like a mistle-toad? Blame the cascade of reactions in your body that alcohol kicks into action.

Alcohol is a diuretic, which means it increases urine production, causing you to go to the restroom more often. This in turn can cause dehydration that leads to a hangover. Alcohol turns off an important messenger in your body called antidiuretic hormone or ADH. ADH tells your body to retain water. Without it, urine becomes very watered down, causing your body to lose water and other important substances such as salt that help regulate the way the cells function. Dehydration throws off the balance of these substances in the brain and contributes to headache symptoms.

Alcohol also causes the buildup of two chemicals called acetaldehyde and acetate. When you drink alcohol, the liver converts the alcohol into acetaldehyde and acetate. Acetaldehyde is much more toxic than alcohol. Too much of it can increase the severity of hangover symptoms. There’s also evidence that the enzyme that breaks down alcohol into acetaldehyde (called alcohol dehydrogenase) may vary between people. These slight differences in how the enzyme works may affect how bad you feel after drinking too much.

The best way to avoid a holiday hangover is to skip alcoholic drinks altogether. But if you decide to indulge, drink in moderation and drink plenty of water to pace yourself and stay hydrated. Also remember to choose a designated driver or have a ride lined up to get home safely.

Happy Holidays!

Audrey A. Vasauskas, PhD, is an associate professor of physiology at the Alabama College of Osteopathic Medicine. She is a former volunteer editor for the I Spy Physiology blog.