Caffeine

What it is

Caffeine is a natural stimulant found in coffee, tea, cocoa, guarana and many soft and energy drinks. It is the most widely used psychoactive substance in the world.

Anhydrous caffeine is the pure, water-free form sold as tablets, capsules or powder. It is used for alertness, workouts and as an ingredient in pre-workouts and fat burners.

Official guidance from EFSA, which UK advice broadly aligns with, suggests up to about 400 mg a day is not a safety concern for most healthy adults, and about 200 mg at a time. Check current guidance, especially for pregnancy, where lower limits are advised.

How it works

Caffeine blocks adenosine receptors in the brain. Adenosine builds up through the day and makes you feel tired, so blocking it increases alertness. It also raises adrenaline and can increase heart rate and blood pressure.

What the evidence says

• Strong Alertness and reduced tiredness
  Reliable for most people, though tolerance builds with regular use.
• Strong Endurance exercise performance
  Consistently shown to improve endurance and reduce perceived effort in many studies.
• Moderate Strength and power
  Small improvements are reported in some studies.
• Moderate Headache relief in combination with painkillers
  Caffeine can boost the effect of some pain relievers.
• Weak Weight loss
  Effects are small and fade with tolerance.

Evidence ratings are our plain-English summary of how much good human research exists, not a promise of results. How we rate evidence.

Dosage and timing

• Typical doses are 50-200 mg, and EFSA suggests up to about 400 mg a day total for healthy adults, with single doses up to around 200 mg.
• Count all sources: coffee, tea, energy drinks, pre-workouts, cola and chocolate.
• Pregnant women are generally advised to keep intake to about 200 mg a day or less; check current NHS guidance.
• Avoid taking it late in the day; its half-life is several hours and it can disrupt sleep.

Forms and what to look for

• Tablets or capsules with a clear dose (often 100-200 mg) are the safest way to dose precisely.
• Coffee, tea and energy drinks vary and contain other compounds.
• Bulk caffeine powder is dangerous: a level teaspoon can contain several grams, a potentially fatal amount, and it is very hard to weigh safely without a milligram scale.
• Check labels for added caffeine in pre-workouts, fat burners and guarana.

What to expect, and when

• Effects begin within roughly 15-45 minutes and last several hours.
• Tolerance develops with daily use, so the same dose has less effect over time.
• Stopping suddenly can cause headaches, tiredness and irritability for a few days.

Side effects

• Jitteriness, anxiety, restlessness.
• Fast or irregular heartbeat, palpitations.
• Trouble sleeping.
• Stomach upset, increased urination.
• Withdrawal headaches when stopping.

Interactions

• Other stimulants (including pre-workouts, decongestants, ADHD medicines): effects add up.
• Some antibiotics (e.g. ciprofloxacin) and fluvoxamine slow caffeine clearance, making it stronger.
• Medicines for heart rhythm or blood pressure: caffeine may oppose or interact.
• Theophylline and lithium: caffeine can affect levels; ask a pharmacist.

Who should avoid it

• People with heart rhythm problems, uncontrolled high blood pressure or anxiety disorders, without medical advice.
• Pregnancy and breastfeeding: stay within lower limits advised by the NHS.
• Children and teenagers.
• Anyone with a history of seizures or severe insomnia.
• Anyone buying bulk powder without precise scales.

Risks and safety

• Overdose: pure caffeine powder has caused deaths, and some countries restrict or warn against bulk powder sales. Symptoms include vomiting, rapid heartbeat, seizures; treat as an emergency (call 999).
• Caffeine is not banned by WADA but is on its monitoring programme.
• Combining several caffeinated products can quickly push intake too high.
• Energy drinks sales to under-16s are restricted by many UK retailers and legislation has been in discussion; check current rules.

Buying well: quality and testing

• Prefer pre-measured tablets or capsules from reputable brands.
• Check mg per tablet and any additional stimulants.
• Avoid bulk powder unless you have a milligram scale and understand the risk.
• Informed Sport or NSF certification matters for athletes.

Bottom line

Caffeine is one of the best-evidenced performance and alertness aids, and also one of the easiest to overdo. Stick to tablets or drinks with known doses, count your total across the day and avoid pure powder. If you get palpitations, anxiety or poor sleep, cutting back is the sensible first step.