Caffeine is a toxin (a poisonous substance produced in living cells) that certain plants produce for defensive reasons. Small amounts of these toxins are not harmful to humans; however, they can have stimulant effects, the user may feel more alert and energised after consumption of this stimulant. Like other drugs (such as cocaine) consuming it in large amounts may cause serious harm.

Caffeine can be found in coffee, tea, cola and energy drinks. Any drinks (not coffee or tea) with a caffeine content of more than 150 mg per litre, have to be labelled as high caffeine content with the amount of caffeine on it (European law).

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Source: https://www.drugscience.org.uk/drugs/stimulants/caffeine