Caffeine is the most commonly used drug in the world. Caffeine prevents the normal binding of adenosine to cellular receptors, thereby inhibiting the reduction of nerve cell activity usually achieved by adenosine binding. Instead, the binding of caffeine to these receptors increases nerve cell activity, stimulating the pituitary gland to release hormones and the adrenal glands to produce adrenaline. This adrenaline increases alertness, heart rate and blood pressure, and the liver releases more sugar into the bloodstream for extra energy. The consumption of caffeine also increases dopamine levels. Dopamine is the "feel good" neurotransmitter that activates pleasure centers in certain parts of the brain. This increased dopamine is likely to contribute to caffeine addiction, in a similar way to the addictive effects of other drugs, such as cocaine.
More than 90% of adults in developed countries consume caffeine regularly, predominantly in the form of coffee or tea. Caffeine is generally recognized as safe, but heavy caffeine use can result in unpleasant side effects such as insomnia, headaches, irritability and nervousness. Heavy caffeine users often become dependent on the drug and can develop a tolerance to caffeine; hence require increasing amounts to obtain the same desired effects.