Caffeine
Caffeine is a stimulant that keeps you alert by blocking adenosine, the brain's sleepiness signal. Healthy adults can have up to 400 mg a day, about four cups of brewed coffee.
400 mga day at most for adults
200 mg in one go, or a day in pregnancy
Under 18 · 3 mg per kg of body weight a day
- Most per 100 g 47mg Dark chocolate
- Most per portion 97mg 1 cup coffee
- Half-life ~4 h on average, from about 2 to 8 h
- Main sources Coffee and tea then cola, energy drinks and chocolate
Limits: EFSA scientific opinion on the safety of caffeine (2015); the FDA uses the same 400 mg figure. Food figures per 100 g from USDA FoodData Central and CIQUAL.
Foods High in Caffeine
Ranked per 100 g, with what a usual portion gives, limited to 3 foods per category. Foods with a Caltide calorie page link to it.
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Per 100 g vs. a Usual Portion
Why the food that ranks first per 100 g can give less in a real serving.
| Food | Usual portion | Per 100 g | Per portion |
|---|---|---|---|
| Coffee | 1 cup (240 g) | 40.6mg | 97mg |
| Cola | 1 can (354 g) | 9.4mg | 33mg |
| Dark chocolate | 2 squares (20 g) | 47mg | 9.4mg |
| Milk chocolate | 2 squares (20 g) | 20mg | 4mg |
| Ice cream | 1 cup (135 g) | 1mg | 1.4mg |
Dark chocolate has more caffeine than coffee per 100 g, but a cup of coffee weighs about 12 times as much as two squares of chocolate, so the cup gives about ten times the caffeine.
How Caffeine Works
Caffeine (1,3,7-trimethylxanthine) is a plant compound found in coffee beans, tea leaves, cacao, kola nuts, guarana and yerba mate. The body has no need for it. It is on this page because it is the most widely used stimulant in the world and it shows up in food labels, drinks and supplements.
While you are awake, a molecule called adenosine builds up in the brain and makes you feel sleepy. Caffeine has a similar shape and sits on adenosine's receptors without switching them on, so the sleepiness signal is muted. It does not add energy; it hides the tiredness that is already there.
How Long Caffeine Stays in Your Body
The gut absorbs nearly all the caffeine in a drink, and blood levels peak within about 30 to 60 minutes. The liver then clears it, mainly through one enzyme, CYP1A2. In healthy adults half of a dose is gone after about 4 hours on average, with a range of roughly 2 to 8 hours between people.
Smoking speeds clearance up. Pregnancy slows it a lot, especially in the last months, and so do birth control pills and medicines that block CYP1A2, such as the antibiotic ciprofloxacin and the antidepressant fluvoxamine. On those, the same coffee lasts much longer.
How Much Caffeine Is Left at Bedtime
Drink 1 cup of brewed coffee from the Caltide library at 3 pm and you take in 97 mg. With a 4 hour half-life, this is roughly what is left as the evening goes on.
| Time | Left in the body |
|---|---|
| 3 pm | ~97mg |
| 7 pm | ~48mg |
| 11 pm | ~24mg |
| 3 am | ~12mg |
EFSA notes that 100 mg close to bedtime can delay sleep and shorten it in some adults. One cup at 3 pm is down to about half by 7 pm and a quarter by 11 pm. A second cup later in the afternoon adds to what is left, and slow clearance, as in pregnancy, keeps far more of it in the blood at bedtime.
How Fast 400 mg of Caffeine Adds Up
Most foods have no caffeine at all, so a day's total comes from a handful of drinks and treats. This sample day uses foods from the Caltide library, in usual portions.
| Meal | Food | Amount | Caffeine |
|---|---|---|---|
| Breakfast | Coffee | 240g | 97mg |
| Breakfast | Oats | 40g | 0mg |
| Lunch | Pizza | 119g | 0mg |
| Lunch | Cola | 354g | 33mg |
| Snack | Dark chocolate | 20g | 9.4mg |
| Snack | Coffee | 240g | 97mg |
| Treat | Ice cream | 135g | 1.4mg |
| Total | 238mg |
That is 238 mg, 60% of the 400 mg adult limit, before any tea, energy drink or pre-workout scoop.
Caffeine With Food and Other Nutrients
Coffee and tea drunk with a meal lower how much plant iron the body absorbs. The polyphenols in the drink do this, not the caffeine itself. People low in iron can drink them between meals instead. Tea also brings theanine, an amino acid studied for how it may shape caffeine's effect on alertness.
Common Caffeine Mistakes
Counting cups, not milligrams. A large takeaway coffee can be twice the size of a 240 ml cup, or more. Count the size of the cup, not the number of them.
Forgetting the hidden sources. Cola, energy drinks, iced tea, chocolate, some pain relievers and pre-workout powders all carry caffeine. Guarana and green tea extract on a label contain caffeine too.
Treating the jitters as normal. A racing heart, shaky hands, anxiety, an upset stomach or poor sleep are the usual signs of too much. Regular drinkers build tolerance, and stopping suddenly can bring a headache and tiredness for a few days.
How Much Caffeine Is Safe
Caffeine has no daily need, only limits. These are the amounts EFSA found raise no safety concern.
| Who | Up to |
|---|---|
| Healthy adults | 400 mg a day |
| A single dose, adults | 200 mg at once |
| Pregnancy | 200 mg a day |
| Breastfeeding | 400 mg a day, 200 mg at once |
| Children and teenagers | 3 mg per kg of body weight a day |
| Close to bedtime | 100 mg can affect sleep |
Caffeine in Supplements
Supplements are a common second source of caffeine, and the easiest one to overdo, because the dose is concentrated.
Names for Caffeine on Supplement Labels
- Caffeine anhydrousplain caffeine with the water removed, the form in most pills and pre-workout powders
- Guarana, green tea extract, yerba mate, kola nutplant extracts that contain caffeine; the label does not always say how much
- Some pain relieversheadache tablets that combine caffeine with a painkiller list it among the active ingredients
Before You Take a Caffeine Product
Add its milligrams to your coffee, not instead of it. One 200 mg pill on top of two large coffees already passes 400 mg. Avoid pure caffeine powder and liquid concentrates: a very small amount holds a very large dose, and kitchen spoons cannot measure it safely.
Ask a doctor or pharmacist before combining a supplement with a medicine.
Tracking Caffeine in Caltide
- Counted against your limitCaltide adds up the caffeine in every food and drink you log, shows the day's total in the Nutrition tab, and flags it once you pass the limit for your age: 400 mg for adults, 3 mg per kg of body weight under 18.
- Pills and powders count tooAdd your caffeine pill or pre-workout once with its caffeine as an ingredient. Each day you check it off, it counts toward your total.
Related Nutrients
Caffeine Sources and References
The figures and claims on this page are checked against these sources. Food amounts come from USDA FoodData Central.
Questions About Caffeine
For healthy adults, more than 400 mg a day. EFSA found no safety concerns up to 400 mg a day and up to 200 mg in one go. In pregnancy the figure is 200 mg a day; while breastfeeding, EFSA found up to 400 mg a day raises no concern for the baby.
Per 100 g, dark chocolate leads the Caltide food library with 47 mg, then Coffee (40.6 mg) and Milk chocolate (20 mg). Per portion, coffee has by far the most, because a cup weighs about 240 g and two squares of chocolate about 20 g.
Brewed coffee in the Caltide library has 40.6 mg per 100 g, so a 240 ml cup holds roughly 100 mg. The real figure depends on the beans, the grind and how strong you brew it.
EFSA puts its half-life at about 4 hours in healthy adults, with a range of roughly 2 to 8 hours, so half of a cup is still in your blood 4 hours later. Smoking shortens it, while pregnancy, birth control pills and some medicines make it longer.
Yes, a little. Decaffeination removes most of the caffeine, not all of it, so several cups of decaf add up to a small amount.
Not at usual intakes. Caffeine has a mild diuretic effect, but in people who drink coffee or tea regularly the water in the drink makes up for it, so these drinks count toward your daily fluids.
EFSA uses 3 mg per kilogram of body weight a day for children and teenagers. For a 50 kg teenager that is 150 mg, about one and a half cups of brewed coffee.
The caffeine is the same molecule. Pills and pre-workout powders state an exact dose, often 100 to 200 mg, which makes them easy to stack on top of coffee without noticing. Pure caffeine powder is the risky one: a tiny scoop can hold a dangerous dose.
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