Whether it’s a morning cup of java, a pre-workout energy drink, or a midday diet cola, there’s a good chance you’re intimately familiar with caffeine. More than just an eye-opener, it can help to increase energy and alertness, and even enhance your performance. Numerous studies have shown that caffeine improves stamina and endurance while lowering perceived exertion and pain, meaning you can work harder yet feel less tired or sore.

But there are a few factors to keep in mind before you start to amp up:

Beware the crash. You know the saying “What goes up must come down”? Caffeine, which stays in your system for about four to six hours, is notorious for causing an early-afternoon crash that can leave you fuzzy headed or fatigued. It doesn’t take much: Anything more than 100 milligrams has been shown to produce a withdrawal. Unfortunately, this up and down can create a cycle in which we turn to more caffeine each day.

If you aren’t already accustomed to caffeine, start with about 50 ­milligrams—about what you might find in a single cup of coffee or a soda—and stick to that if it’s enough to make a difference in your workout. If you’re already downing more than that, slowly cut back for about a week till you’re at a level that doesn’t make you feel dependent on caffeine just to function.

Keep an eye on your intake. Too much caffeine can cause a rapid heart rate, shaking hands, or a feeling of anxiety or nervousness. Over the long term you may also notice more cramping or tightness, particularly in the hands, forearms, or calves. Coffee and colas aren’t as likely to cause this reaction, but certain energy drinks or pre-workout supplements may. In general, try to keep your caffeine dose below 300 milligrams a day. Caffeine is a mild diuretic, but much like its stimulant effect, it’s less noticeable over time. To be safe, make sure you’re properly hydrated.

Be careful what it’s paired with. Most pre-workout energy supplements include caffeine in one form or another, as well as other, more exotic, herbal extracts. These can provide a synergistic effect that offers a major lift. But in some people—especially those who are sensitive to caffeine or have a preexisting condition like heart disease—they can cause a more extreme reaction. So read labels carefully, and if you’re not used to caffeine, start with just half a serving and gauge your reaction accordingly.