You may feel fine hitting the trail or gym after one or two drinks—heck, you may feel great—but there's more going on in your body than you probably realize. "Some research has shown that small amounts of alcohol actually increase muscular endurance and strength output—but these benefits are very short-lived," says exercise physiologist and nutrition scientist Stacy Sims, Ph.D. "After 20 minutes or so of exertion, the negative effects start to surface, and these easily outweigh the positives."
Some of these effects are obvious: reduced coordination and reaction time, for example, which put you at risk for injury. Even if you're not buzzed, though, boozing interferes with your body's ability to burn carbs and fat, because your liver is working to metabolize alcohol above anything else. This means you're not getting the energy you need to maintain a normal activity level, and your performance will undoubtedly suffer.
What's more concerning, though, is the fact that drinking causes blood vessels to dilate and rush blood to the skin's surface, in order to offload excess heat. "Coupled with alcohol's dehydration effects, this compromises your body's ability to thermoregulate," says Sims. This can result in unpleasant side effects like increased heart rate, excess perspiration, and stomach cramps, to name a few. At the same time, blood is diverted away from your muscles, reducing their strength and endurance capacity. (And while we're on the topic: No, you can't sweat out booze.)
Finally, alcohol causes inflammation throughout the body, so muscles are weaker and sorer afterward, and take longer to recover—which may have a negative effect on your next workout, too. Your best bet, says Sims, is to wait 12 hours between drinking and any vigorous activity. In the meantime, sleep it off and hydrate, hydrate, hydrate.
Bottom line: Once in a while, booze-fueled exercise won't kill you—but keep it brief, go easy on the intensity, and in general, don't do it. Schedule your workouts for first thing, and save the beer (just one, though!) for après-sweat.