Side Effects Of Pre-Workout That Might Surprise You

Athletes and gym-goers spent a whopping 12.6 billion on pre-workout supplements in 2019, reports Grand View Research. Most pre-workout formulas boast a combination of caffeine, creatine, beta-alanine, branched-chain amino acids (BCAAs), and other nutrients. Caffeine, for example, can increase your energy and mental focus. Creatine fuels physical performance, while BCCAs reduce muscle breakdown and aid in recovery. Some products may also contain taurine, citrulline, and other amino acids, points out Grand View Research.

Most people use pre-workout supplements for a quick boost of energy. But the benefits don't stop here. These products can also improve body composition (aka fat-to-muscle ratio), enhance running performance, and delay fatigue, suggests a 2010 study featured in the Journal of the International Society of Sports Nutrition (JISSN). As the researchers note, caffeine alone can reduce fatigue and increase endurance.

These perks come at a price, though. For some folks, the side effects of pre-workout supplements outweigh any potential benefits. Since these products are not regulated by the FDA, you don't really know what's inside the bottle (per Grand View Research). Manufacturers often use proprietary blends, meaning that they don't disclose how much of each ingredient is used. To stay on the safe side, check the label and read customer reviews before buying new products. 

Your pre-workout may cause digestive distress

Pre-workout supplements are generally safe in the short run, explains a 2019 study published in Nutrients. However, more research is needed to confirm their long-term safety. But even so, certain ingredients in these products have potential side effects. Magnesium, for example, increases bowel motility by drawing water into the intestines, notes Verywell Health. Therefore, it may cause loose stools or diarrhea when consumed in large amounts.

Caffeine has similar effects. Drinking two or more cups of coffee daily can cause diarrhea (per the International Foundation for Gastrointestinal Disorders). Some pre-workout supplements contain highly concentrated caffeine in liquid or powder form. BSN Endorush, for instance, delivers 200 milligrams of caffeine per serving, whereas a cup of brewed coffee has 96 grams of caffeine (via BSN Supplements and the Mayo Clinic). Therefore, one serving of BSN Endorush has more than double the amount of caffeine in one cup of java and can give you the runs.

Creatine, another popular ingredient in pre-workout supplements, may cause diarrhea too. This side effect is more likely to occur when you ingest over 10 grams of creatine in one sitting, reports a 2008 review featured in Research in Sports Medicine

You may gain water weight

Speaking of creatine, did you know that it may cause water retention? This amino acid pulls water into the muscle cells, causing them to expand. But you may also notice an increase in body weight due to the extra water, states a 2007 review published in the JISSN. On the positive side, your muscles may appear fuller and recover faster from training.

Anecdotal evidence suggests that creatine can also cause bloating, muscle cramps, dehydration, and organ damage (via the JISSN). However, current research doesn't support these assertions. Creatine has no detrimental effects on healthy people, points out the JISSN. On the contrary, it may enhance athletic performance and reduce injury risk. In clinical trials, it has been shown to increase maximal power and strength by 5 to 15% and sprint performance by 1 to 5%, reports the JISSN. Moreover, it promotes muscle growth and recovery.

If you're concerned about weight gain, you can take a pre-workout supplement without creatine. However, water weight gain is temporary and shouldn't affect your workouts — or your health.

You might experience headaches

Your pre-workout might also give you a headache, says registered dietician Lonnie Lowery, Ph.D. (via Men's Journal). In general, this side effect is due to arginine, an amino acid with vasodilating properties. After ingestion, arginine dilates and relaxes the blood vessels in your head (and other tissues), which in turn may cause headaches. Other vasodilators, such as citrulline, may have similar effects.

L-citrulline, a naturally occurring amino acid, is converted to arginine in your body and therefore expands the blood vessels, explains a 2021 research paper published in Applied Sciences. However, both arginine and citrulline support optimal health and may improve sports performance. When used as part of an exercise program, these compounds may delay fatigue and increase your speed, strength, and muscle oxygen uptake. They also have beneficial effects on metabolism, cardiovascular function, and sexual health (per Applied Sciences).

The JISSN states that most pre-workout supplements are safe and have minimal adverse effects. However, some products may contain hormones, banned substances, or impurities. That's why it's important to choose a trusted brand and research the ingredients used. Pay attention to the amount of caffeine per serving, too. When consumed in excess, this stimulant may cause nausea, headaches, irregular heartbeat, and other side effects, notes the JISSN. You might also want to check the Operation Supplement Safety (OPSS) database, which features a list of potentially harmful supplements and ingredients, along with practical tips to help you make the right choice.