A legal stimulant sits in millions of kitchen cabinets, locker rooms and convenience-store coolers. Taken at the right dose, it may boost your performance in a race.

A review of 48 clinical trials found that even modest caffeine doses can help endurance athletes finish faster, with the largest gains appearing at moderate doses. But a separate sprint study adds an important warning: athletes who drink a lot of coffee every day may get less of a boost when they use caffeine before competing. The findings suggest that caffeine can help, but the best dose may depend on both the race and the athlete’s daily habits.

Caffeine is a psychoactive drug. It stimulates the central nervous system, reducing fatigue and drowsiness. During physical exercise, that can make hard work feel easier and help athletes hold a faster pace.

A new meta-analysis focused on sports time trials, where the question is simple: How long does it take to finish a race? Most of the evidence came from cycling, which accounted for 33 of the 48 studies. Running made up 13 studies, with rowing and other sports barely represented.

At 1.3 to 3 milligrams of caffeine per kilogram of body weight—the equivalent of a strong coffee for a 70-kilogram athlete—the trials showed a significant improvement in aerobic time-trial performance. In practical terms, that was enough to reach the study’s threshold for a meaningful speed gain, roughly around 2%.

Moderate doses performed better. At 4 to 6 mg/kg, athletes finished about 2.18% faster on average, and the effect appeared more reliable across trials. The authors wrote that moderate doses “appear to produce a more consistent ergogenic effect.” An ergogenic effect refers to the enhancement of physical power, mental capability, or athletic performance.

But the response varied. A statistical measure showed wide disagreement across studies, meaning caffeine helped some athletes more than others.

A separate study added daily habits to that equation. Researchers at Dublin City University tested 18 male team-sport athletes across ten 40-meter sprints, using caffeinated gum equal to about two strong cups of coffee. Men with low daily caffeine use maintained performance better. Men who already drank more than 130 mg/day—around three or more cups a day—did not get the same lift and declined over repeated sprints.

The message is practical: more caffeine does not automatically mean better performance.

The endurance review found benefits at low doses and stronger average effects at moderate doses. But it found no qualifying time-trial studies that tested doses above 6 mg/kg. That leaves the high-dose zone—where some athletes may chase an extra boost—poorly understood.

There are also obvious reasons to be cautious. Higher caffeine intake can bring jitters, heart palpitations, and stomach trouble. For a runner or cyclist, gut distress can erase any performance gain.

The evidence also has gaps. The endurance studies were heavily male-biased, with women making up only about 7% of participants. The sprint study included only 18 men. Genetics, body size, sleep, training status, and daily caffeine habits may all shape the response.

For athletes, the safest lesson is to test caffeine during training, not on race day. A small dose around 2 to 3 mg/kg may be enough to help. Some may benefit from 4 to 6 mg/kg. Regular heavy coffee drinkers may need to cut back before competition to feel the same effect.

Caffeine cannot replace training, pacing, or recovery. The sweet spot may be enough to feel alert, not enough to rattle the body.

The studies were published in Nutrients and International Journal of Sport Nutrition and Exercise Metabolism.