Caffeine

Established Supplementsperformance cognition Last reviewed 2026-09-24

Summary

A central nervous system stimulant and the most widely studied ergogenic aid in existence. Trial evidence supports modest improvements in endurance performance and reduced perceived exertion at moderate acute doses.

Mechanism

Caffeine primarily blocks adenosine receptors in the brain and spinal cord, reducing perceived exertion and fatigue. Secondary peripheral effects on muscle calcium handling have been proposed, but the central nervous system mechanism is the best supported.

Reported effects

Ratings are relative to peers in the same category, not absolute claims.

Aerobic endurance performance★★★★☆Most consistent benefit: ~2-3% improvement in time-trial performance at 3-6 mg/kg.

Perceived exertion★★★☆☆Reduces ratings of perceived exertion during exercise by roughly 5-6%.

Muscle strength and power★★☆☆☆Ergogenic effects reported but smaller and less consistent than for endurance.

Studied dosing — research context

Trials most commonly use an acute dose of 3-6 mg/kg taken about 60 minutes before exercise, typically in capsule form. The minimal effective dose may be around 2 mg/kg; 9 mg/kg adds side effects without clear extra benefit. These are doses used in research, not guidance.

Research context only. This is not a recommendation.

Safety

Reported side effects include jitteriness and anxiety, gastrointestinal upset, sleep disruption (especially when taken later in the day), and acute increases in blood pressure. Doses around 9 mg/kg carry a high incidence of side effects without clear additional performance benefit over 3-6 mg/kg.

Limitations

Most primary studies were conducted in young men; women and older adults are underrepresented. Individual response varies substantially with genetics (e.g., CYP1A2) and habitual use, and genotype-based personalization claims remain contested rather than settled.