Has your skin ever tingled during your workout? It might be because of the ingredients in certain supplements.

Many people turn to “pre-workout” supplements to get the most out of their time in the gym. The supplement, sold in powder form and usually mixed with water, is taken prior to your workout with hopes of increasing your energy level, endurance, and focus.

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There’s a wide range of brands and flavors, but many of them contain similar ingredients: caffeine, taurine, beta-alanine, and creatine.

Thanks to the amino acid beta-alanine, you may experience a numbing sensation in your skin, similar to the feeling you get when your arm or leg “falls asleep” after being in the same position for an extended period of time. This pins and needles feeling is known as paresthesia.

Some products, like this one, even warn users this may happen. “Some individuals may experience a harmless tingling sensation, which is attributed to beta alanine. USE ONLY AS DIRECTED,” the label reads.

Beta-alanine is believed to affect your muscles by creating a molecule called carnosine, which is said to help counteract lactic acid buildup; this helps you push through challenging workouts, host Hank Green explains in SciShow's video above.

In addition to hanging out in your muscles, the amino acid also occupies your nerves. Researchers believe when the beta-alanine combines with specific nerve receptors, it stimulates them, causing you to feel that tingly sensation.

Although researchers aren’t certain what molecular mechanisms cause paresthesia, they have noticed it seems to be dose-dependent. Some people may be able to handle a certain amount without side effects, while others may not.

See also: 6 Workout Supplements You Should (And Shouldn’t) Pack In Your Gym Bag

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