Caffeine is a naturally occurring substance found in the leaves, seeds or fruits of at least 100 different plant species worldwide and is part of a group of compounds known as methylxanthines. The most commonly known sources of caffeine are coffee, cocoa beans, cola nuts and tea leaves. Caffeine is also added to specifically formulated 'energy drinks' and pharmaceutical products such as cold and flu remedies.

Coffee and tea also contain other dimethylxanthines – theophylline, which has similar properties to caffeine and theobromine, whose pharmacological action is far less potent than caffeine and theophylline.

The amount of caffeine present in products depends on the type of the product, the serving size and the preparation method. For example a 190ml cup of tea contains 50mg of caffeine, one third less than the same amount of a cup of instant coffee (75mg). The table below gives an indication of the amount of caffeine found in other drinks compared to tea:

On average we consume 3.98mg of caffeine /kg body weight per day i.e. 239mg/day for a 60kg person.