How much caffeine is too much? 400 mg is the number, and it is a soft one.

For most adults, the US Food and Drug Administration cites 400 milligrams of caffeine a day — about two to three 12-fluid-ounce cups of coffee — as an amount not generally associated with negative effects [s1]. That figure is not a safety cliff and was never derived as one. The agency states alongside it that there is wide variation both in how sensitive people are to caffeine and in how fast they clear it, and that what counts as too much depends on body weight, medications, medical conditions and individual sensitivity [s1].

Where 400 milligrams comes from

The number originates in a Health Canada assessment published in 2003 and was re-tested in a systematic review published in 2017 [s2]. That review screened more than 5,000 articles, included 381 meeting its criteria across five outcome types — acute toxicity, cardiovascular toxicity, bone and calcium effects, behaviour, and development and reproduction — in four healthy populations, and assessed each against the Health Canada comparators of 400 mg/day for adults, 300 mg/day for pregnant women and 2.5 mg/kg/day for children and adolescents [s2].

Its conclusion for adults was that when study quality, consistency, level of adversity and magnitude of response were weighed together, the evidence generally supports that consumption of up to 400 mg of caffeine a day in healthy adults is not associated with overt adverse cardiovascular, behavioural, reproductive, developmental, acute or bone effects [s2]. The FDA cites this review as confirming the 400 mg level for most adults, and identifies its authorship as the Institute for the Advancement of Food and Nutrition Sciences, a food-industry-linked research body — a provenance worth knowing when the headline figure in question is a food-safety threshold [s1].

Two qualifiers in that sentence do real work. "Healthy adults" excludes the people most likely to be affected. "Overt, adverse" sets a bar that a night of poor sleep or a raised resting heart rate does not clear.

Pregnancy, where the number is different and older

The evidence base for pregnancy is thinner and the guidance is more conservative. The 2017 review concluded that up to 300 mg a day in healthy pregnant women is generally not associated with adverse reproductive and developmental effects [s2]. The American College of Obstetricians and Gynecologists sets a lower reference point: moderate caffeine consumption, defined as less than 200 mg per day, does not appear to be a major contributing factor in miscarriage or preterm birth [s3]. The same document is explicit about what remains open — the relationship of caffeine to fetal growth restriction remains undetermined, and no final conclusion can be drawn about whether high caffeine intake and miscarriage are correlated [s3]. Committee Opinion No. 462 dates from August 2010 and was reaffirmed in 2026 [s3].

Children, where there is barely a number at all

The 2017 review found limited data for children and adolescents, and concluded only that the available evidence suggests 2.5 mg per kilogram of body weight per day remains an appropriate recommendation [s2] — a recommendation carried forward rather than newly established. The FDA notes that medical experts advise against energy drinks for children and teens because of their sugar and caffeine levels, and that too much caffeine in this group can cause increased heart rate, palpitations, high blood pressure, anxiety, sleep problems, digestive problems and dehydration [s1].

What is actually in a drink

The FDA publishes typical caffeine content per 12 fluid ounces: 23 to 83 mg in a caffeinated soft drink, 37 mg in green tea, 71 mg in black tea, 113 to 247 mg in regular brewed non-specialty coffee, and 41 to 246 mg in an energy drink [s1]. Energy drinks generally contain 54 to 328 mg per 16 fluid ounces [s1]. Decaffeinated coffee is not caffeine-free — typically 2 to 15 mg in an 8-fluid-ounce cup [s1].

The ranges are the point. A single 12-ounce coffee can supply anywhere from a quarter to more than half of the 400 mg figure depending on how it was brewed, and restaurants and retail food establishments are not required by law to disclose caffeine content [s1].

Where the real danger sits

Not in coffee. The FDA estimates that toxic effects such as seizures can be observed with rapid consumption of around 1,200 mg of caffeine — less than half a teaspoon of pure caffeine [s1]. That is the exposure that pure and highly concentrated caffeine products created when they appeared in bulk packaging with up to thousands of servings per container, requiring consumers to measure a safe serving out of what could be a toxic or lethal quantity [s1]. The FDA took action against those products, though some remain on the market [s1].

Signs of excess intake listed by the agency are increased heart rate, palpitations, high blood pressure, insomnia or sleep disruption, anxiety, jitters, upset stomach, nausea and headache [s1]. It adds that caffeine withdrawal is not considered dangerous but can be unpleasant, and that anyone cutting back is best doing so gradually [s1].

This article is informational and is not medical advice. Caffeine interacts with a range of medications and medical conditions, and the FDA's own guidance is to discuss intake with a health care provider if you are pregnant, trying to become pregnant, breastfeeding, or managing a condition or medication that affects caffeine sensitivity [s1].

Sources

• Spilling the Beans: How Much Caffeine is Too Much? — US Food and Drug Administration, content current as of 2024-08-28
• Systematic review of the potential adverse effects of caffeine consumption in healthy adults, pregnant women, adolescents, and children — Food and Chemical Toxicology, 2017
• Moderate Caffeine Consumption During Pregnancy (Committee Opinion No. 462) — American College of Obstetricians and Gynecologists, August 2010, reaffirmed 2026