Caffeine

What it is

Caffeine is a stimulant found in coffee and other foods and drinks [1], and sold as pills and pre-workout supplements [2]. A typical cup of brewed coffee contains about 107 mg per 250 mL [2].

What the research says

When people are short of sleep, caffeine improves attention and reaction speed. A 2020 meta-analysis of 45 publications found improved response time and accuracy on attention tests [3]. In a trial of 20 special forces personnel, repeated 200 mg doses kept reaction speed and vigilance steadier than placebo across three nights without sleep [4].

For rested people, the evidence is smaller. A 2021 review found significant gains only for attention, accuracy and speed, in five pooled studies [5]. A 2018 trial found that regular coffee reduced reaction time compared with placebo in young and older adults [6].

For endurance, a 2018 meta-analysis of 46 studies found moderate doses (3 to 6 mg per kg) improved time-trial performance, with completion time about 2.2% faster [7]. A 2026 meta-analysis of 48 studies found low and moderate doses shortened time-trial times [8]. A 2020 umbrella review of 21 meta-analyses supported benefits for aerobic endurance, muscle strength and muscle endurance [9].

For strength, a 2021 meta-analysis found benefits for bench press repetitions and maximum bench strength but not leg press [10]. A 2022 meta-analysis found doses of 0.9 to 2 mg per kg improved strength and endurance in resistance exercise [11].

Doses used in studies

Endurance trials used moderate doses of 3 to 6 mg per kg of body weight [7], and a 2026 review found low doses of 3 mg per kg or less also shortened time-trial times [8]. Strength trials used 0.9 to 2 mg per kg [11]. Sleep-deprivation trials used repeated 200 mg doses, 800 mg in total per day [4]. Panic-disorder trials used 400 to 750 mg [12].

Safety and interactions

Common side effects in athletes are heart palpitations and poorer sleep onset, and they rise with dose [13]. Caffeine reduced total sleep time by about 45 minutes across 24 studies [2]. In people with panic disorder, caffeine caused panic attacks in about 51% of patients at 400 to 750 mg [12]. A systematic review of caffeine safety found up to 400 mg a day is not linked to overt harm in healthy adults, and up to 300 mg a day in pregnant women [14]. Higher intake during pregnancy was linked to pregnancy loss in observational studies [15]. Adverse-event reporting in adolescent athletes was inconsistent, and the authors say routine use in youth sport is not supported [16]. No upper intake limit was reported in the trials cited here, and no data were found on interactions with medicines or on breastfeeding.

Caveats and open questions

Many trials used athletes or sleep-deprived people, so results may not apply to everyday use [3][13]. The evidence for focus in rested people rests on few, small trials [5]. Most individual studies were conducted in young men [9]. Two registered trials are ongoing: one tests caffeine on chess performance and cognition (NCT07641335), and one compares optimised with standard caffeine dosing (NCT05588934).