Gym bag, check. Car keys, check. Coffee downed, check. Yes, a caffeine kick could be a valuable addition to your pre-exercise routine, delaying muscle fatigue and keeping you focused and energetic. You don’t want to overdo it, though.  Sleep problems, headaches, irregular heartbeat, high blood pressure, or maybe even a heart attack can result. Here’s how to work caffeine into your workouts.

Match the amount to your body. “The larger you are, the more metabolically active tissue you have,” says Nicholas Gant, director of the Exercise Nutrition and Metabolism Laboratory at the University of Auckland in New Zealand. “If you’re a small person, your tissues don’t use up as much, therefore you need a lesser dose.” A very rough recommendation is 0.5 to 1.4 milligrams of caffeine per pound of body weight. Coffee averages about 20 mg per ounce, or 160 mg per 8-ounce cup. That’s about the limit for a 130-pound woman, though a 200-pound could probably down a couple of cups. Go above 4 mg of caffeine per pound and your workout could be ruined by digestive distress, the jitters, and other unpleasant side effects.

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Track your tolerance. Going by body weight alone doesn’t take individual tolerance into account. If you’re a caffeine newbie, a smaller dose will initially provide a noticeable difference. If coffee is already part of your daily routine, you’re likely to need more for the same effect. For someone training for a marathon or other serious athletic event, says Gant, who advises many New Zealand Olympians, reducing caffeine from all sources—tea, soda, and foods like chocolate and energy bars in addition to coffee—can dramatically improve sleep quality, decrease blood levels of cortisol (a stress marker), and increase your body’s caffeine sensitivity to give you an extra energy boost when you do imbibe just before the event. “Only use it on those few occasions that you’re really going to need that pick-me-up,” says Gant.

Go by the clock (and calendar). If coffee is programmed into your exercise schedule, drinking it 30 to 60 minutes before you start will put your blood level of caffeine at a peak during your workout. If you are a coffee regular and a big event is coming, like a marathon or 100-mile bike race, Gant recommends that you gradually taper down the week before, so that a modest amount the day of competition will get you going.

Add fuel to the fire. Getting juiced on caffeine doesn’t put gas in the tank. Registered dietician Leslie Bonci, director of sports nutrition at the UPMC Center for Sports Medicine in Pittsburgh, recommends pairing your coffee with oatmeal or a small bagel with peanut butter before you exercise to avoid cramps. “Drinking coffee black with a small snack provides the fluid you need for hydration as well as the carbohydrate, sugar and sodium that the body needs to sustain itself,” she says.

Consider the source. Because, the caffeine content of coffee can vary widely, it’s hard to know how much you’re actually taking in. Substituting for caffeine-spiked sports drinks like Gatorade or Powerade, says Gant, lets you monitor your intake more precisely just by reading the nutritional label. But don’t confuse such drinks, which contain sugar and other carbs to keep the energy going, with sugar-free, caffeine-loaded “energy drinks” like Red Bull Sugarfree. The caffeine rush will be brief (and expensive)—”a lot of money for a flash of energy that won’t be sustained,” says Gant.

Don’t sweat dehydration. Too much coffee and your body will suffer because caffeine is a diuretic and make you lose vital fluid—or so goes the belief. It’s not true, says Bonci. “It’s a common misconception that when you consume caffeine sometimes you feel the urge to void,” she says. “But in studies that looked at the urine volume produced over 24 hours whether or not someone consumed caffeine, there’s not a significant difference.” The problem with the misconception is that it leads many a coffee drinker to compensate by drinking much more water than necessary, and that could send any jogger running for the nearest bush.