Coffee and caffeine: what the evidence says, and how much is too much

For most healthy adults, FDA has cited 400 mg of caffeine a day, about two to three 12-ounce cups of coffee, as an amount not generally associated with negative effects. People vary widely in how sensitive they are and how fast they clear caffeine, and some conditions and medicines make people more sensitiveFDA. Signs of too much include a fast heart rate, palpitations, raised blood pressure, sleep problems, anxiety, jitters, upset stomach, nausea and headache FDA.

Strength varies a lot by brew, cup size and bean, so "cups" are a rough guide. Energy drinks, pre-workout powders and some over-the-counter medicines add to the total.

Caffeine works by blocking adenosine receptors in the brain Mclellan 2016. A review of its effects found that low to moderate doses reliably improve alertness, vigilance, attention and reaction time, while effects on memory and higher-level thinking such as judgment are less consistentMclellan 2016. Physical performance improved at doses above about 200 mg, and repeated doses helped people keep performing during restricted sleep Mclellan 2016.

That is a short-term effect on how you feel and perform, not a fix for being short of sleep night after night. If you rely on coffee to get through every day, the sleep page is the place to start.

In a controlled study, 400 mg of caffeine taken at bedtime, three hours before or six hours before bed all disturbed sleep compared with placebo. The authors concluded that people should avoid substantial caffeine for at least six hours before bed Drake 2013. An afternoon coffee can still be cutting into your night even if you fall asleep without trouble.

A 2017 umbrella review in the BMJ gathered 201 meta-analyses of observational research and 17 of interventional research. Coffee drinking was more often linked with benefit than harm. The largest relative risk reductions were at three to four cups a day versus none: 17% lower all-cause mortality, 19% lower cardiovascular mortality and 15% lower cardiovascular disease Poole 2017.

The authors were clear about the limits: these are associations, and randomized trials are needed to show whether coffee causes any of them. Two exceptions stood out. In pregnancy, higher coffee intake was linked with low birth weight, preterm birth and pregnancy loss, and in women it was linked with a higher fracture risk Poole 2017. Those are why this page grades the long-term findings limited and does not suggest starting coffee for your health.

In people with high blood pressure, 200 to 300 mg of caffeine raised blood pressure by about 8 mm Hg systolic and 6 mm Hg diastolic, starting within an hour and lasting at least three hours. Over two weeks of regular coffee, though, blood pressure did not rise compared with decaf or a caffeine-free diet, and cohort studies found no link with heart disease in this group Mesas 2011.

How you brew matters for cholesterol. A meta-analysis of 14 trials found unfiltered coffee, such as boiled, French press or Scandinavian-style, raised total and LDL cholesterol, while filtered coffee caused very little increase Jee 2001.

Caffeine can make anxiety worse. In a meta-analysis of challenge studies in people with panic disorder, about half had a panic attack after a large dose of caffeine (400 to 750 mg, roughly five cups), compared with under 2% of healthy volunteers. Healthy people also felt more anxious at those dosesKlevebrant 2022. If you live with anxiety or panic attacks, cutting back is a reasonable thing to try and to discuss with your clinician.