Most healthy adults can handle about 400 milligrams of caffeine a day. That is the figure the European Food Safety Authority gave after reviewing the evidence. The US Food and Drug Administration uses the same number.
It sounds simple. In practice a cup is not a fixed dose.
What 400 milligrams looks like
A 240-millilitre (8-ounce) cup of brewed coffee holds about 80 to 100 milligrams. A single espresso holds about 60 to 75 milligrams. A large takeaway cup can hold 300 milligrams or more.
That means four ordinary cups sit near the limit. One large cold brew may use most of it.
Single doses matter
EFSA also looked at single doses. It found that up to 200 milligrams at once does not raise safety concerns. That is roughly two cups.
Larger single doses can cause jitters, a fast heartbeat and poor sleep. Timing matters as well. Caffeine has a half-life of about five hours in most adults.
Who should drink less
Pregnant women are advised to keep intake below 200 milligrams a day. Children and teenagers are advised to take far less. People with heart rhythm problems or anxiety should talk to a doctor.
Genes also play a part. Some people clear caffeine quickly. Others feel one cup for hours.
The right amount is the one that lets you sleep.
A practical rule
Track what you drink for a week. Add up the cups, the size and the type. Stop caffeine about eight hours before bed.
If you feel shaky, cut back slowly. Quitting fast can cause headaches.
