Many pre-workout supplements contain ingredients that can cause digestive problems, spike your heart rate to unsafe levels, or simply waste your money. Some even contain substances the FDA has declared illegal. Knowing what to scan for on the label (and what should send a product straight back to the shelf) can save you from side effects that range from unpleasant to genuinely dangerous.
Proprietary Blends
A “proprietary blend” is a list of ingredients that only shows the total weight of the blend, not how much of each ingredient is actually in it. Manufacturers are legally required to name every ingredient in the blend but not the individual amounts. That means a pre-workout could list caffeine, beta-alanine, and citrulline in a 5-gram blend, and you’d have no way of knowing whether you’re getting 300 mg of caffeine or 50 mg. One ingredient could make up nearly the entire blend while everything else is present in trace, ineffective amounts.
This matters for two reasons. First, you can’t assess safety. If you’re trying to stay under a daily caffeine limit, for example, a proprietary blend that includes coffee bean extract, guarana, and green tea extract gives you no way to calculate your total caffeine intake. Second, you can’t assess effectiveness. Research on ingredients like citrulline and creatine shows benefits at specific doses. Without knowing how much you’re getting, you’re guessing. Look for products with fully transparent labels that list every ingredient with its exact dose per serving.
Illegal Stimulants Like DMAA
DMAA (1,3-dimethylamylamine) is a powerful stimulant that the FDA has declared illegal to sell as a dietary supplement. It is not a legitimate dietary ingredient, and despite sometimes being marketed as a “natural” extract from geranium plants, the FDA says there is no reliable evidence that DMAA naturally occurs in plants. It has been linked to serious cardiovascular events, including heart attacks and strokes.
The problem is that DMAA still shows up in pre-workout products, sometimes under alternative names designed to slip past a quick label check. Watch for any of these on an ingredient list:
- 1,3-DMAA or 1,3-Dimethylamylamine
- Methylhexanamine or Methylhexanenamine
- Geranamine or Geranium extract
- Pelargonium graveolens extract
- 2-Amino-4-methylhexane
- 4-Methyl-2-hexanamine
If you see any of these, skip the product entirely. A related stimulant called DMHA (octodrine) raises similar concerns and is worth avoiding for the same reasons.
Excessive Caffeine
Caffeine is the most common active ingredient in pre-workouts, and in reasonable doses it genuinely improves performance. The issue is quantity. The FDA considers 400 mg per day a safe ceiling for most healthy adults. Some pre-workout products pack 300 to 400 mg into a single serving, which leaves almost no room for the coffee, tea, or soda you might also drink that day.
Too much caffeine causes jitteriness, anxiety, a racing heart, digestive upset, and disrupted sleep. If you work out in the afternoon or evening, even a moderate-caffeine pre-workout can interfere with sleep quality, which undermines the recovery your training depends on. Check the caffeine content per serving and factor in everything else you consume throughout the day. If a product doesn’t list its caffeine dose clearly (often hidden inside a proprietary blend), that alone is reason to pass on it.
Stimulant Stacking With Medications
High-stimulant pre-workouts become especially risky if you take medications that already affect your heart rate or blood pressure. Stimulant medications for ADHD, such as amphetamine-based drugs, already raise heart rate, elevate blood pressure, and increase body temperature during exercise. Layering a high-caffeine pre-workout on top of that compounds every one of those effects, raising the risk of dangerous overheating, severe anxiety, and cardiac events.
Common decongestants also increase heart rate and blood pressure. If you’re taking one for a cold and still plan to train, adding a stimulant pre-workout creates a similar stacking problem. The same caution applies to anyone on blood pressure medication, since stimulants can work directly against what that medication is trying to do.
Artificial Food Dyes
Many pre-workouts get their bright colors from synthetic dyes like Red 40, Yellow 5, and Yellow 6. These three dyes have been found to be contaminated with benzidine and other carcinogens. At least four common food dyes, including Blue 1, Red 40, Yellow 5, and Yellow 6, are known to cause hypersensitivity reactions in some people. Yellow 5 in particular has shown positive results for genotoxicity (DNA damage) in multiple lab studies.
Artificial dyes serve zero functional purpose in a pre-workout. They don’t improve performance, taste, or mixability. They exist purely for visual appeal. Products that skip them entirely are easy to find and give you one less unnecessary additive to think about.
Maltodextrin and Cheap Fillers
Maltodextrin is a common filler and bulking agent in pre-workout powders. It’s a processed carbohydrate with a glycemic index above 90, which is higher than table sugar (around 65). Consuming it before exercise triggers a sharp insulin spike. In research on cyclists and runners, ingesting maltodextrin before exercise caused significantly elevated insulin levels at the start of training, followed by a transient drop in blood sugar during the workout itself. That blood sugar crash can leave you feeling lightheaded or fatigued right when you’re trying to perform.
If you see maltodextrin high on an ingredient list, it likely means the product is using it as cheap filler to bulk up the serving size. A quality pre-workout delivers its active ingredients without needing to pad the scoop.
Sugar Alcohols in High Doses
Sugar alcohols like sorbitol, xylitol, and maltitol are used as sweeteners in many pre-workout formulas. In small amounts, they’re generally fine. In larger doses, they pull water into the intestines and cause bloating, gas, cramping, and diarrhea. The threshold varies by type.
Sorbitol and mannitol are the most likely to cause problems, with digestive symptoms starting at just 10 to 20 grams in some people. Xylitol is better tolerated, with most people handling 10 to 30 grams in a single dose without diarrhea, though 50 grams in one sitting significantly increases nausea, bloating, and watery stools. Erythritol is the gentlest option: doses under 20 to 35 grams typically cause no noticeable symptoms.
Pre-workout servings rarely contain enough of any single sugar alcohol to hit those upper limits, but if you’re taking other supplements, chewing sugar-free gum, or consuming protein bars throughout the day, the amounts add up. If you notice digestive issues during workouts, sugar alcohols in your pre-workout are a likely culprit.
Artificial Sweeteners
Sucralose and acesulfame potassium (ace-K) are the two most common zero-calorie sweeteners in pre-workouts. Research on their long-term effects is still evolving, but animal studies have raised flags worth knowing about. In mice, four weeks of ace-K consumption significantly altered gut bacterial composition, with the effects differing between males and females. Male mice showed increased body weight gain, higher levels of a key energy metabolism compound, and changes to bile acid ratios in the gut. Sucralose has similarly been shown to impair the growth of beneficial gut bacteria in rats.
These are animal studies, and the doses don’t translate directly to humans. But if you’re consuming artificial sweeteners across multiple products every day (pre-workout, protein powder, flavored water, diet soda), your cumulative intake is worth considering. Products sweetened with stevia or monk fruit offer alternatives if you’d rather avoid synthetic sweeteners altogether.
Unverified Brands and Heavy Metals
Dietary supplements in the U.S. don’t require FDA approval before hitting store shelves, which means quality control varies enormously between brands. When the Clean Label Project tested 133 protein powder supplements in 2018, every single product contained detectable levels of heavy metals. Seventy percent contained measurable lead, and 74% contained measurable cadmium. Pre-workout powders face the same manufacturing and sourcing risks.
The most reliable safeguard is choosing products that carry a third-party certification. Programs like NSF Certified for Sport, Informed Sport, and BSCG test for banned substances, verify that the label matches what’s actually in the product, and audit manufacturing facilities for compliance with good manufacturing practices. These certifications require testing in ISO 17025 accredited laboratories and include toxicology assessments of every ingredient. A product without any third-party certification isn’t automatically dangerous, but one that carries a recognized certification gives you a meaningful layer of assurance that an unverified brand simply can’t.
High-Dose Beta-Alanine
Beta-alanine is a legitimate performance ingredient, but it’s worth understanding its main side effect before you encounter it mid-workout. Single doses above roughly 0.8 grams commonly trigger paresthesia: an intense tingling or prickling sensation across the skin, often on the face, neck, and hands. It’s harmless, but if you’ve never experienced it, it can feel alarming.
Many pre-workouts contain 1.5 to 3.2 grams of beta-alanine per serving, well above the tingling threshold. If you find the sensation uncomfortable, look for products with a lower dose or split your serving in half and take it in two portions about 30 minutes apart. The performance benefits of beta-alanine come from consistent daily use over weeks, not from a single large dose, so splitting it up doesn’t cost you anything.