Caffeine
The stimulant in coffee and tea, sold as tablets for alertness and training.
- Alertness and focus
- Endurance and stamina
- Strength and power
- Sprinting and team sports
A plain-English overview, not advice. Written from published reviews and trials and from what people say in public forums.
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On this page
What is caffeine?
Caffeine is a plant stimulant, and the world’s most widely taken psychoactive substance. It occurs naturally in coffee, tea and chocolate, and is also made synthetically and added to food and drink. Sold on its own it is usually caffeine anhydrous, simply the dried powder, pressed into tablets or weighed out loose, so that a serving is a known amount rather than a guess. What draws people to it is concentration and memory on the one hand, and physical performance on the other.
Why do people take caffeine?
Alertness and focus
The thinking. This is the oldest reason of all, and the commonest: a tablet before a night shift, a long drive or a late revision session. The thinking is that caffeine muffles the brain’s own sleepiness signal, so attention holds steadier for longer, including when sleep has been short.
What research has found. Two trials here measured thinking rather than exercise. One gave caffeine straight after a learning task and found better recall a day later, with a moderate amount working better than a larger one. The other kept people awake for thirty hours and found quicker reaction times, though the slide in sustained vigilance was not held back. Studies here point in different directions, so this one is still being worked out; the dose, how long people took it and who took part all seem to matter.
Endurance and stamina
The thinking. Runners, cyclists and swimmers take caffeine about an hour before a hard session or a race, most often 3 to 6 milligrams for every kilogram they weigh. The thinking has less to do with the legs than the head: the same pace registers as a shade easier, so it can be held for longer.
What research has found. Reviews of time trials found small but fairly consistent gains, a few per cent, and rated aerobic work the setting where caffeine shows up most reliably. Trials in cyclists found the same, including in people tired out mentally beforehand. Two saw the gain shrink after weeks of daily use, and one found it differed by a gene that sets how fast caffeine clears. Research here is still young: a few studies so far, mostly pointing the same way.
Strength and power
The thinking. In the gym caffeine is taken before a session in the hope of moving the bar faster and finding another repetition or two. The thinking is about how hard the set feels rather than the muscle itself: trials that recorded effort ratings alongside lifting found them lower on caffeine.
What research has found. Reviews of lifting and of short all-out sprints found small gains, roughly 3 to 4 per cent on power. The amount mattered: 3 milligrams per kilogram moved light loads but not the heaviest, where 9 was needed, and that larger amount brought far more unwanted effects. In athletes already used to a lot of caffeine daily, 9 and 11 milligrams per kilogram changed neither strength nor repetitions. Only a few studies have measured this so far, though they mostly agree.
Sprinting and team sports
The thinking. Footballers, basketball players and others whose sport comes in bursts take caffeine shortly before playing, for the tenth sprint as much as the first. The thinking is that jumping, sprinting again and again and changing direction all draw on the same short, hard efforts.
What research has found. A review pooling 34 studies of team sports found small but significant gains in jump height, sprint speed and agility, and, during matches themselves, more total running, more sprinting and more distance covered at speed. A trial in 18 women footballers taking 3 milligrams per kilogram found the same pattern in a simulated match. This is early work, a small number of studies, mostly pointing the same way.
What does caffeine do in the body?
Adenosine is a chemical that gathers in the brain across a long day, and as it gathers it makes you feel sleepy. Caffeine fits the same receptors without switching them on, so while it is there the sleepiness signal is muffled. That is why reviews describe its effects as central ones: what changes is how tired and how worked you feel, not the muscle.
At larger amounts it does other things too, releasing calcium inside cells and blocking an enzyme called phosphodiesterase. Almost all of it is then broken down in the liver by a single enzyme, and how fast that happens varies a great deal from person to person: age, sex and hormones, smoking, weight, liver health and diet all shift it, and so do two common gene variants, one setting the speed of clearance and one how sensitive the receptors are. That variation is why the same tablet can leave one person sharp and another jittery.
What do people say about taking caffeine?
These are anecdotes people have shared, not evidence.
What people like
- Very commonly, endurance racers describe a clear lift in drive and mood partway through a long race, and hanging on with faster training partners.
- Several people like knowing exactly how much they have taken, and a few turn to tablets simply because they do not drink coffee at all.
- People who cut caffeine out for a week or more beforehand often say the same tablet then feels far more noticeable on race day.
- A minority keep tablets strictly for races rather than training, and say that is what keeps the effect worth having.
Common complaints
- Jitteriness, shakiness and anxiety are by far the most common complaint, with several saying they felt wired but no better able to concentrate.
- An urgent need for the toilet during hard running comes up repeatedly, and posters say it is worse the faster they go.
- A racing heart, raised blood pressure and broken sleep are mentioned often, along with a heavy crash an hour or two later.
- Withdrawal headaches during a deliberate break are commonly described, typically around the third to sixth day without any caffeine at all.
- Some people notice nothing at all, or only tiredness, and a few say tablets did less for them as use became regular.
How people take it
- The usual pattern is one 200 milligram tablet taken 30 to 120 minutes before a session or race, rather than every day.
- Halving a tablet is a common way in for anyone who found a whole one too strong, or who is trying it first time.
- Endurance racers commonly spread doses across a long event, roughly 200 milligrams at a time, working to about 3 to 6 milligrams per kilogram.
- Going without caffeine entirely for four days to a month beforehand is widely discussed, though many abandon it as too disruptive.
- A strongly repeated rule is to try tablets in training first and never to use one for the first time on race day.
Read from these public discussions:
- Is it ok to keep taking caffeine tablets? (Mumsnet, 2023)
- Took a 200 mg caffeine pill on my 10 mile run = diarrhea (Slowtwitch triathlon forum, 2012)
- Caffeine pills (Slowtwitch triathlon forum, 2007)
- Do you give up caffeine before races? (Slowtwitch triathlon forum, 2015)
- Caffeine pills (AnandTech forums, 2005)
What forms of caffeine are there?
| Form | What’s different | Per serving, on products we list |
|---|---|---|
| Caffeine | Caffeine anhydrous is caffeine with the water removed: a dried powder, pressed into tablets or sold loose. | 200 mg |
What else is worth knowing about caffeine?
- Labels give caffeine per serving, and a serving may be more than one tablet. The trials behind this page mostly used 3 to 6 milligrams for every kilogram of body weight, taken about an hour beforehand; a common tablet is 200 milligrams. Larger amounts, around 9 milligrams per kilogram, brought far more unwanted effects without being needed for the effect.
- Caffeine disturbs sleep, and from earlier in the day than most people expect. Pooling 24 studies, one review found total sleep time about 45 minutes shorter and deep sleep about 11 minutes shorter, and calculated that a 217 milligram serving would need more than 13 hours before bed to leave sleep untouched. A trial giving 400 milligrams at bedtime, three hours before and six hours before found all three disturbed sleep against a dummy.
- Across 25 studies in athletes, a racing heart and difficulty dropping off were the most common and the strongest of the unwanted effects, and they were more frequent above about 6 milligrams per kilogram. Two reviews in healthy adults found anxiety scores higher on caffeine than without it, particularly above about 400 milligrams.
- The effect can fade with regular use. One trial found the endurance gain gone after four weeks of a small daily dose, and another watched it shrink over 20 days of daily capsules.
- Most of this research was done in young men, and reviews of women’s sport are newer and fewer. A broad safety review of healthy adults concluded that intakes up to 400 milligrams a day were not generally linked with harm, and put the figure at 300 milligrams in pregnancy. Caffeine is also broken down by a liver enzyme that handles many prescribed medicines, and interacts with a number of them.
If you take medication, are pregnant, or have a health condition, check with a pharmacist or GP first.
The research behind this page (22 studies)
- Post-study caffeine administration enhances memory consolidation in humans. (2014, randomised trial)
- A comparison of the effects of transcranial direct current stimulation and caffeine on vigilance and cognitive performance during extended wakefulness. (2014, randomised trial)
- Wake up and smell the coffee: caffeine supplementation and exercise performance-an umbrella review of 21 published meta-analyses. (2020, meta-analysis)
- The Effect of Acute Caffeine Ingestion on Endurance Performance: A Systematic Review and Meta-Analysis. (2018, meta-analysis)
- Time course of tolerance to the performance benefits of caffeine. (2019, randomised trial)
- Caffeine improved cycling trial performance in mentally fatigued cyclists, regardless of alterations in prefrontal cortex activation. (2019, randomised trial)
- Caffeine, CYP1A2 Genotype, and Endurance Performance in Athletes. (2018, randomised trial)
- Chronic ingestion of a low dose of caffeine induces tolerance to the performance benefits of caffeine. (2017, randomised trial)
- Placebo in sports nutrition: a proof-of-principle study involving caffeine supplementation. (2017, randomised trial)
- Caffeine ingestion enhances Wingate performance: a meta-analysis. (2018, meta-analysis)
- Effects of caffeine intake on muscle strength and power: a systematic review and meta-analysis. (2018, meta-analysis)
- The Effects of High Doses of Caffeine on Maximal Strength and Muscular Endurance in Athletes Habituated to Caffeine. (2019, randomised trial)
- Caffeine ingestion acutely enhances muscular strength and power but not muscular endurance in resistance-trained men. (2017, randomised trial)
- Neuromuscular responses to incremental caffeine doses: performance and side effects. (2013, randomised trial)
- Effects of acute ingestion of caffeine on team sports performance: a systematic review and meta-analysis. (2019, meta-analysis)
- Caffeine-containing energy drink improves physical performance in female soccer players. (2014, randomised trial)
- International society of sports nutrition position stand: caffeine and exercise performance. (2021, review)
- Interindividual Differences in Caffeine Metabolism and Factors Driving Caffeine Consumption. (2018, review)
- The Safety of Ingested Caffeine: A Comprehensive Review. (2017, review)
- Caffeine: cognitive and physical performance enhancer or psychoactive drug? (2015, review)
- Adenosine, caffeine, and sleep-wake regulation: state of the science and perspectives. (2022, review)
- The neuroprotective effects of caffeine in neurodegenerative diseases. (2017, review)
How much does caffeine cost?
That depends on the form and the pack, so we work out the price of the same dose of every caffeine product we can find and rank them.