In 2011, Army Private Michael Sparling was on a run while training with his unit. He had taken a pre-workout supplement prior to exercising, and within 10 minutes he found himself passed out, collapsed, in cardiac arrest, and later died. The elevation of his heart rate due to the combined effects of the supplement and the running pushed his body beyond what it was capable of, and his passing is a tragedy.
There are too many reports of medical emergencies caused by ingestion of pre-workout supplements to be ignored, and there are too many consumers buying these products without looking at the labels. Most pre-workouts contain about 300 milligrams of caffeine, the equivalent of about four cups of coffee. Not only will this elevate your heart rate to an abnormal level, but it will leave you with little to no energy for the rest of the day.
When taken with taurine (another common ingredient), caffeine can even accelerate instead of delay muscle fatigue. Some also contain agmatine sulfate. If you're taking pre-workout, chances are that you're also taking creatine, and it's not recommended to take these two supplements together. Additionally, almost all pre-workouts contain beta alanine. There is evidence showing that beta-alanine can postpone one's feeling of exhaustion during high-intensity training, but normal to high doses of this drug often overstimulates nerve cells and causes a scary, itchy, tingling sensation throughout the skin known as paresthesia. I've only mentioned a few ingredients, but I could go on and on.
In my opinion, the worst ingredient commonly found in pre-workout supplements is L-carnitine. L-carnitine is an amino acid that, when taken in large doses, not only damages your liver and kidneys but causes a horrible fishy body odor. I experimented with L-carnitine about a year ago and stopped taking it within a week just because of the fishy body odor. Please, I'm begging you, don't put yourself through that. It's terrible.
If you do read the labels and still choose to purchase these supplements, you should know that a lot of the advertised effects aren't entirely accurate. Nootropics, drugs that claim to increase an individual's mental awareness and focus, are a common ingredient in pre-workouts. While you may be able to find a couple of studies out there to back up these claims, the reality is that no significant evidence exists to support the validity of nootropics.
Ultimately, the additional energy and one or two reps in your sets are not worth the many, MANY health problems associated with pre-workout supplements. As long as you are getting a solid amount of complex carbohydrates in your diet, there's no need for these products.