Where does caffeine act, and does it raise adrenaline?
Caffeine passes from the stomach and gut into the blood, then up to the brain, where it sits on the receptors of adenosine, the molecule that signals tiredness, without switching them on: tiredness is felt less. In someone who does not usually drink it, it also raises adrenaline; in regular drinkers, this effect almost disappears within a few days.
Sources: IOM 2001 (absorption, liver, receptors), Robertson 1978 (250 mg in 9 people who do not drink coffee: adrenaline +207%), Robertson 1981 (effect almost gone after 1 to 4 days of regular intake), Lovallo 2005 (cortisol). For regular drinkers, the small rise shown is our own estimate.
Frequently asked questions about coffee and the heart
Why do I get palpitations after a coffee?
Often, these are extrasystoles: a beat comes early, then a short pause feels like a "skipped" beat. In a 2023 study of 100 adults followed day by day, coffee days had about 154 extrasystoles from the ventricles per day, versus 102 on days without coffee. Those from the atria barely changed (58 versus 53). These early beats are usually harmless in a healthy person.
Why can coffee speed up the heart?
Three reasons add up. Caffeine blocks some of the adenosine receptors, which also act as a small brake on the heart (their exact role in everyday palpitations is still poorly known). In people who drink little, it raises adrenaline. Finally, some people clear caffeine slowly (pregnancy, the pill, liver genes), so it stays longer and higher in the blood.
Is an energy drink the same as a coffee?
Not quite. A large can often has as much caffeine as a strong coffee or more, is drunk fast and cold, sometimes with other stimulants and lots of sugar. Several cans in a row make a big dose all at once, and that kind of dose is what most often makes the heart race.
Does coffee cause heart rhythm problems in the long run?
In a large British study of over 380,000 people (UK Biobank), coffee drinkers had no more rhythm problems than others; each daily cup even went with a slightly lower risk (Kim, 2021). This is an observation, not proof that coffee protects.
When should I see a doctor?
If the heart beats fast or irregularly with chest pain, faintness, shortness of breath or loss of consciousness, call emergency services. Palpitations that come back often, even without coffee, are worth discussing with a doctor.