1. May make you feel jittery

Caffeine is one of the primary ingredients in many pre-workout supplements.

This stimulant has been shown to increase muscle strength and output during exercise while reducing fatigue (1Trusted Source2Trusted Source3Trusted Source).

In theory, caffeine allows you to get more out of a given workout.

Nonetheless, caffeine has several potential side effects, especially if you consume too much. These include insomnia, nausea, increased heart rate, drowsiness, headaches, anxiety, and jitteriness or restlessness (4Trusted Source).

What’s more, many pre-workout supplements pack high amounts — up to 500 mg of caffeine per serving. Serving sizes typically range from 0.35–1 ounce (10–30 grams).

In comparison, 1 cup (240 ml) of coffee contains only 95 mg.

Ways to reduce side effects

Caffeine dosing is very individualized, as some people tolerate it better than others.

The best way to reduce side effects is to start with a small dose of a caffeinated pre-workout supplement, slowly increasing your dosage to see what you can tolerate.

Of course, you can also choose pre-workout supplements without any caffeine.


2. May increase water retention

Another popular ingredient in many pre-workout formulas is creatine.

It has been shown to increase high-intensity exercise capacity and lean body mass gains from exercise (6Trusted Source).

While it’s most often part of a pre-workout supplement, creatine can also be taken on its own.

The main side effects associated with creatine are fairly mild but include water retention, bloating, weight gain, and digestive issues.

Ways to reduce side effects

Despite these side effects, creatine has been shown to be exceptionally safe (7Trusted Source8Trusted Source).

You can reduce any adverse symptoms by ensuring proper dosing.

Creatine is typically dosed with a loading phase of 4 scoops (20 grams) per day for at least 3 days, followed by a 3–5-gram daily maintenance dose.

This method provides rapid benefits — but has a higher potential to cause digestive issues and bloating (6Trusted Source).

Alternatively, you can take a single daily dose of 3–6 grams if you’re willing to wait for 3–4 weeks to experience benefits. This option is best if you want to avoid side effects like bloating, especially for those with a sensitive stomach (6Trusted Source).

Notably, it may be difficult to avoid moderate weight gain of 2–6 pounds (1–3 kg) while taking creatine. This is mainly due to increased water retention in your muscles