An unexpected side effect of aging isn’t gray hair or wrinkles, but the inability to drink two glasses of wine without feeling like you’ve been hit by a truck the next morning.

Back in the days of youth, you may have been able to down a few drinks — or more — without feeling the consequences. But add a couple decades and you might need more than a day to recuperate.

When you drink, enzymes in your liver break the alcohol down into acetaldehyde (asset-all-de-hyde). Acetaldehyde, which is toxic, is then broken down again into acetate. When you drink more than your liver can process, acetaldehyde runs rampant in your body.

When you’re younger, your liver may be able to keep up in a timely fashion. But as you age, your liver produces fewer of the enzymes that can break down acetaldehyde. If you drink the same amount at 45 as you did at 25, your body may not be able to keep up, spreading acetaldehyde throughout your body. Acetaldehyde can also induce inflammation throughout your digestive tract and brain — causing queasiness and that pounding headache.

Your 25-year-old immune system may have been able to tamp this inflammation down before it ruined your whole day, but your 45-year-old immune system may be a little weakened by age.

The key to avoiding hangovers is to simply avoid alcohol — at least to excess. The National Institute on Alcohol Abuse and Alcoholism defines low-risk drinking as no more than three drinks on any single day or no more than seven per week for women, and no more than four drinks on a single day and no more than 14 drinks per week for men.

When you do drink, replace your second beer with a glass of water. Sounds boring, but your future self may thank you for it.