What are the effects of caffeine?

Stimulant: Caffeine's effects often depend on the individual. For some people, it can provide energy, focus, and alertness. In others, and especially at higher concentrations, it can cause jitteriness, problems sleeping, and palpitations. With repetitive use, you may become tolerant to caffeine, meaning its effects are reduced over time.

Many: It is a stimulant, chemically similar to amphetamine but much less potent. It can cause anxiety, tremor, insomnia and irritability, and in some very sensitive people, this can be severe and include elevation of blood pressure and pulse. In high doses, like in some of the energy drinks, some of these symptoms have had serious consequences, with a few deaths, probably due to heart arrhythmias.