Caffeine occurs naturally in the leaves, seeds and fruits of more than 60 plants, including coffee, tea leaves, kola nuts and cocoa beans. Caffeine narrows blood vessels in the brain and stimulates the cerebral cortex, the brain's outer sheath. This helps you think more rapidly and clearly. Elsewhere in the body, blood vessels expand.
Caffeine is used medically as a mild stimulant or headache-killer.
Caffeine citrate helps in treating sleeping problems in newborns. In healthy people, moderate amounts of caffeine have little effect on blood pressure or heart rate. Bigger doses can cause agitation and ringing in the ears, muscle tremors and irregular heartbeat. If you quit coffee cold, you may experience headache, nervousness, anxiety dizziness and irritability. Remember, though, coffee isn't a drug.