Jitters mean the dose outran your sensitivity.
Caffeine blocks the brain's sleepiness signal and raises adrenaline; the FDA lists jitters among too-much symptoms, with anxiety, insomnia, fast heart rate, palpitations, upset stomach, nausea, and headache. Sensitivity and metabolism speed vary widely between people, so there is no universal safe cup count, only the FDA's 400 milligram daily guide for most adults. Empty stomachs, stacked sources like energy drinks, and genetics all lower the threshold. Eat first, downsize, or move to tea; chest pain, irregular heartbeat, or faintness with jitters is a GP matter.
What to do
Common questions
Why do I get jitters from coffee?
Jitters are caffeine doing exactly what it is designed to do, just past your personal dose. Caffeine blocks adenosine, the brain's sleepiness signal, and pushes up adrenaline; at the right dose that feels like alertness, and past it, it feels like trembling hands, a racing mind, and a thumping heart. The FDA lists jitters among the signs of too much caffeine, alongside anxiety, insomnia, increased heart rate, heart palpitations, upset stomach, nausea, and headache. The catch is that your dose and my dose differ: the FDA says there is wide variation in how sensitive people are to caffeine and how fast they metabolize it, which is why your colleague drinks espresso at 4pm and sleeps fine while one flat white makes you vibrate. An empty stomach, energy drinks or pre-workout stacked with coffee, some medicines, and plain genetics all lower your threshold. The fixes: eat first, shrink the serving or switch to tea, stop caffeine by early afternoon if sleep suffers, and count all sources, not just coffee. If jitters arrive with chest pain, an irregular-feeling heartbeat, or faintness, or if tiny amounts set them off reliably, mention it to your GP rather than just switching to decaf.