How much caffeine is too much in a day?

The working numbers, all from the European Food Safety Authority and matched by the US FDA:

• 400 mg a day for a healthy adult
• 200 mg in a single sitting
• 200 mg a day during pregnancy or breastfeeding
• 3 mg per kg of body weight for adolescents, so about 175 mg for a 58 kg teenager

400 mg sounds generous until you count what you actually drink.

Try it with your drink

Half-life 5.7 h · peak at 45 min · sleep threshold 40 mg

Drink

When you had it

Bedtime

Metabolism

Fast Average Slow

At bedtime

—mg

Below 40 mg at

—

Latest for tonight

—

Cira runs this on every drink you log and tells you the hour to stop. Get Cira free →

What 400 mg looks like

The pattern is that coffee gets you there slowly and energy drinks get you there in two moves. Two cans of Celsius is the entire daily limit with nothing left for the morning coffee.

A 400 mg day is not dangerous. It is the point above which the evidence for harmless stops being clear, which is a different claim.

The 200 mg single dose

The daily total is not the only limit that matters. EFSA sets a separate figure for one sitting, because the effects that bother people come from the peak rather than the average.

Above roughly 200 mg at once, a meaningful share of people get a raised heart rate, restlessness, shaky hands and the flat anxious feeling that gets called the jitters. Below it, most do not. If it has already happened, here is how to get caffeine out of your system. Two espressos back to back, a caffeine tablet, a pre-workout scoop and most large energy drinks all sit at or above that line.

Spreading the same total across the day avoids almost all of it. 400 mg as four cups two hours apart feels nothing like 400 mg as two energy drinks at lunch.

Who should sit lower

• Pregnancy and breastfeeding: 200 mg. Clearance slows to as much as 15 hours in the third trimester, so a single afternoon coffee behaves like a much larger dose.
• Anxiety disorders and panic attacks: caffeine reliably worsens both, and the threshold is individual and often well below 400 mg.
• Arrhythmias, uncontrolled hypertension: worth a conversation with a doctor rather than a rule of thumb.
• Children: no safe level is established, and health bodies advise against caffeinated drinks entirely.
• Anyone taking fluvoxamine, ciprofloxacin or similar: these inhibit the enzyme that clears caffeine, sometimes several-fold. A normal coffee becomes an unusually large one.

Tolerance is not permission

Regular drinkers stop feeling much from a cup within days to weeks, because the brain grows extra adenosine receptors to compensate. This is genuine and it is also narrow: it dulls the subjective buzz, not the blood pressure response, not the effect on sleep architecture, and not the clearance rate.

So "400 mg does nothing to me" usually means the alerting effect has faded, while the same 400 mg still shortens your deep sleep if the last of it lands after your cutoff. If you have built the tolerance and get the alertness back only above 400 mg, the honest read is that the drug has stopped working, and a two week reset restores it better than a larger dose.

The overdose question

Real caffeine toxicity needs roughly 1,200 mg in one go: the US FDA puts toxic effects such as seizures at about that amount taken quickly, and lethal doses are far higher again. You cannot get there on coffee: it would take twelve mugs in an hour, and you would be too nauseous long before.

Every serious case comes from concentrated products. Anhydrous caffeine powder is the one to genuinely avoid, since a teaspoon can hold several grams, and a handful of tablets or highly dosed pre-workout is the other realistic route. Energy drinks, coffee and tea are simply not concentrated enough to be dangerous in the volumes anyone drinks.

A practical daily shape

If you want a rule that survives contact with real days: put the biggest dose before 11 am, keep any single serving at or under 200 mg, stop by your own cutoff rather than a fixed hour, and count energy drinks, tea, cola and pre-workout in the same total as coffee. The daily number takes care of itself after that.