Caffeine Before a Workout Might Have Dangerous Side Effects

Your high-octane sports drink is a great performance-enhancing drug. Multiple studies have linked caffeine to improved endurance and strength.

In a 2019 study, Raúl Domínguez, PhD, leader of the sports physiology laboratory at the University Isabel I in Spain, found athletes who took caffeine supplements maximized their performance in resistance workouts.

Domínguez suspects caffeine increases lipid metabolism, breaking down fats to generate energy; activates more muscle fibers and stimulates the sympathetic nervous system, decreasing the perception of pain. Taken together, it appears caffeine allows you to push harder to achieve fitness gains — and experienced athletes might enjoy the biggest boost.

“The most interesting thing about caffeine is this supplement [can enhance physical performance] in the majority of sport modalities but it seems to have a higher effect in athletes,” Domínguez says. “Caffeine supplementation could be effective to increase sports performance, which could have very interesting impacts for training.”

While the connections between caffeine intake and performance benefits are clear, those gains could come with a significant side effect: A new study published in Medicine & Science in Sports & Exercise found using caffeine before or during a workout could increase the risk of blood clots.

The researchers followed 48 men across two high-intensity training sessions (spaced one week apart) and found caffeine increased their coagulation factor during exercise. Translation: The men who had a caffeinated drink before exercising were at higher risk for developing clots, which are linked to serious (and sometimes fatal) health issues such as stroke, heart attack, deep vein thrombosis and pulmonary embolism.

The men in the study had a normal BMI and an average age of 23; they were given a dose of caffeine equal to 5mg per kilogram of body weight (about 340mg of caffeine — or 3.5 cups of coffee — for a 150-pound man).

One possible explanation for the effect is caffeine and exercise both affect blood flow to the liver, which synthesizes some of the enzymes responsible for coagulation, notes study co-author Paul Nagelkirk, PhD, associate professor and director of the integrative exercise physiology lab at Ball State University.

Nagelkirk admits more research is needed to understand exactly how using caffeine before a workout increases the risk of clotting. In the meantime, he is not suggesting ditching your pre-workout cup of coffee or skipping caffeinated energy gels during long runs.

“One thing about caffeine is that people have very individualized responses to it: Some can comfortably metabolize large amounts while others are only able to tolerate very little or none at all,” he says. “It’s reasonable to think that caffeine would influence clotting potential in some individuals more than others but this has not yet been studied scientifically.”

If you have cardiovascular risk factors such as high blood pressure, high cholesterol, obesity or tobacco use, you may be more likely to experience adverse effects of using caffeine before a workout, Nagelkirk says.

“Caffeine is safe for most people … and healthy adults who currently enjoy the benefits of caffeine as a pre-workout or pre-competition routine have little reason to worry,” he adds.