What is a pre-workout supplement?

In one line

A pre-workout is a supplement that mixes several ingredients, such as caffeine, into one product meant to be taken just before exercise to help the workout.

What is a pre-workout supplement?
A supplement made to be taken just before exercise
At the gym, you often see people taking a supplement before they train. This post explains what a pre-workout is, which ingredients it usually contains, and what is useful to know when reading its label, step by step.
In one line
A 'before exercise' supplement that mixes many ingredients
'Pre-workout' means 'before a workout'. It refers to a supplement that mixes several ingredients into one product, meant to be taken just before exercise to help the workout. There is no set recipe, so ingredients differ a lot from product to product.
Most common ingredient 1: caffeine
Found in about 9 of 10 top-selling products
About 9 of 10 top-selling pre-workouts contained caffeine. One serving held about 250 mg on average, roughly two cups of coffee. Caffeine is the ingredient that blocks the brain's sleepiness signal and makes you feel more awake.
Most common ingredient 2: beta-alanine
An ingredient that raises 'carnosine' in muscle
An ingredient as common as caffeine is beta-alanine. Beta-alanine raises a substance called 'carnosine' in muscle. Carnosine is a substance that eases the shift toward acid inside muscle during hard exercise.
The next most common ingredients
Citrulline, tyrosine, taurine, creatine
The next most common ingredients were citrulline, tyrosine, taurine, and creatine, in that order. Citrulline becomes raw material in the body for a signal that widens blood vessels. Creatine is an ingredient that quickly refills muscle energy when you produce short, hard bursts of force.
Ingredients without amounts
Nearly half the ingredients had no amount listed
One product contained about 18 ingredients on average. Nearly half of them were grouped under a 'blend' name, with only the total weight of the blend listed and not the amount of each ingredient. US rules only require ingredients in a blend to be listed in order from most to least.
Compared with amounts used in studies
Beta-alanine mostly fell short of study amounts
Among products that listed amounts, beta-alanine averaged 2 g per serving, half of 4 g a day, the lowest amount that worked in studies. Of the products that listed beta-alanine, only one went above the study amount. For a 70 kg person, caffeine was close to the low end of the amounts used in studies.
Ingredients not on the label
A hidden stimulant has been found
In one workout supplement, a stimulant similar to methamphetamine that was not on the label was found. This stimulant had never once been studied in people. In the US, the government does not approve supplements for effect and safety before they are sold.
Why it tingles
It is a side effect of beta-alanine
The tingling feeling on the skin after taking it is a side effect of beta-alanine. It was the only side effect reported in beta-alanine studies. The benefit of beta-alanine appeared only after taking it steadily for several weeks so that carnosine built up in muscle.
A common misconception
Does it contain exactly the amount on the label?
No. When the caffeine in pre-workouts sold in Australia was measured directly, products ranged from far less than the label to almost twice the label. Even the same product differed from batch to batch, and more than half of the products did not list their caffeine amount at all.
Summary
A pre-exercise supplement that mixes many ingredients
A pre-workout is a supplement that mixes many ingredients, such as caffeine and beta-alanine, to be taken just before exercise, and ingredients and amounts differ a lot between products. When caffeine was measured directly, the amount on the label and the amount inside differed a lot. Related terms: caffeine, beta-alanine, citrulline.