The vast majority of pre-workouts contain stimulants. And when a product is said to contain stimulant, this usually refers to the use of caffeine. This natural chemical is thought to boost mental alertness, increase strength and raise energy and endurance – all of which could help improve performance in the gym. Caffeine comes in different forms. Caffeine anhydrous is thought to be stronger and more ergogenic than the natural caffeine found in coffee or tea. Dicaffeine malate (a blend of caffeine and malic acid) is said to reduce “digestive stress”, while caffeine citrate is marketed as a quicker way to increase blood caffeine levels for faster-acting results in the gym.