How much caffeine is actually safe in a day?

Two numbers dominate the conversation, and both come from the same 2015 scientific opinion published by the European Food Safety Authority: about 400 mg a day from all sources for healthy adults, and about 200 mg in a single dose. UK guidance and most national bodies land in a similar place.

For pregnancy the advised figure is lower, at around 200 mg a day, and that is the number the NHS uses. For children and adolescents the guidance is generally expressed relative to body weight rather than as a flat figure, which makes sense given the range of body sizes involved.

It is worth being clear about what "not a safety concern" means. It is a statement about population risk assessed against specific endpoints. It is not a promise that 400 mg will suit you, and it is certainly not a recommendation to consume that much.

The single most common way people underestimate their intake is by counting only the thing they think of as caffeinated. A realistic tally includes:

• Coffee, in all its forms, including decaf, which is reduced rather than caffeine-free.
• Tea, especially black, green and matcha.
• Cola and other soft drinks that contain added caffeine.
• Energy drinks and energy shots.
• Pre-workout powders, which are frequently the largest single source in a day for people who use them.
• Dark chocolate and cocoa, in smaller amounts.
• Some over-the-counter painkillers, particularly combination headache and cold remedies.

Packaged drinks are the easy part, because in the UK a drink above a set caffeine threshold has to declare the amount on the label. Coffee from a shop is the hard part, because the amount varies enormously with the size and the preparation and nothing is printed on the cup.

Caffeine is broken down in the liver, and how quickly that happens varies considerably between individuals. Part of that variation is genetic. On top of that, several everyday factors shift it:

• Regular use builds tolerance to some of the effects, so a habitual drinker and an occasional one are not comparable.
• Body size plays a part, which is why guidance for children is weight-based.
• Pregnancy slows clearance considerably, which is part of why the advised limit is lower.
• Smoking speeds it up; stopping smoking slows it back down, which occasionally catches people out.
• Several medications interact with caffeine metabolism in both directions, and a pharmacist can check yours.
• Anxiety sensitivity differs, and caffeine can amplify symptoms in people prone to them.

For most people, the practical problem with caffeine is not the daily amount but the hour of the last dose. The commonly quoted half-life is somewhere around five hours, meaning roughly half of a dose is still in circulation five hours later, though that figure varies widely, and half of a large dose is still a meaningful amount.

The consequence is that an afternoon drink can still be working at bedtime, and sleep can be affected even on nights when falling asleep feels normal. Worse sleep produces a more tired next day, which produces more caffeine, which is how a sensible habit turns into an escalating one.

These are the effects people most commonly report at doses above what they are used to: jitteriness or shakiness, a racing or pounding heart, restlessness, an upset stomach, feeling anxious or on edge, needing to urinate more often, and difficulty getting to sleep. They generally pass as the caffeine clears.

If you get chest pain, a persistent irregular heartbeat, repeated vomiting or you feel faint, that is a reason to seek medical help rather than to wait it out.

• Add up an ordinary day honestly, using labelled figures wherever they exist.
• If the total surprises you, reduce it gradually rather than stopping abruptly, because withdrawal headaches are unpleasant and avoidable.
• Set a cut-off time before you set a daily limit. It usually does more for how you feel.
• Keep one or two lower-caffeine days a week so tolerance does not quietly climb.
• Judge it on sleep and on how you feel in the late afternoon, not on how the first hour goes.