Pre-workout supplements include all the ingredients that people take with the singular goal of making their workouts more effective. The name literally means it's something you do prior to a workout, game, or match.
"In the hour prior to training, you should be focused on priming your internal environment for hard physical—and mental—work," explains Dwayne Jackson, Ph.D., in the article "The Science Behind the Best Pre- and Post-Workout Supps." "Therefore, pre-workout supplementation should optimize your mind and body to maximize performance."
However, that "optimization" can mean several things. In many cases, pre-workout supplements provide extra energy or focus. But they can also impart specific physical benefits. For example, one type of athlete might appreciate the increased pain tolerance or force-production it provides during an all-out effort, like a heavy rep test on the "big three" during Layne Norton's PH3 Power and Hypertrophy program. Another might appreciate the endurance to knock out a few more reps in high-volume DTP sets in Kris Gethin's 8-Week Hardcore Trainer. Vasodilators and other ingredients can help increase blood flow to muscle tissue, boosting recovery and endurance.
The most common ingredients in today's popular pre-workouts include caffeine, beta-alanine, leucine or BCAAs, and nitric oxide boosters such as Nitrosigine, arginine, or citrulline, all of which appear to help increase blood flow and the "pump" feeling during lifting. However, new blends with new ingredients come out all the time.
Pre-workouts can be divided into two groups: those with stimulants, and those without. The stimulants pretty much always include caffeine, in widely varying doses, but may include other stimulants including yohimbine, theacrine (Teacrine as the most common), guarana, and many others. Many supplement companies have developed stimulant-free pre-workouts for those who are sensitive to caffeine, simply don't want it, or who exercise close to bedtime.
If you're looking for a pre-workout that will make you feel amped up and ready for action, you'll find no shortage of them out there. However, if that's exactly what you don't want, you still have choices. And many pre-workouts fall in the middle, with no more caffeine than a cup of tea or coffee.
All of this is why it's essential to read labels, read plenty of user reviews, and buy pre-workouts from established, reputable companies.