A crystalline compound produced for its pesticidal properties by a number of plant species as a deterrent to predators, caffeine acts as a central nervous system stimulant when ingested by humans. It is both fat- and water-soluble, and its consumption in a variety of preparations is widespread, with records of deliberate usage for its psychoactive effects dating back centuries. In addition to enhancing athletic performance and endurance, caffeine reduces feelings of sleepiness and improves concentration. Effective dosage is determined by the user’s physiology and tolerance, and can vary enormously among a population. Caffeine’s stimulant properties are expressed through multiple mechanisms of action, but the bulk of its activity takes place through its antagonist effect on certain neurotransmitter receptors. It binds to and obstructs the receptors for adenosine, a neurotransmitter which accumulates in the brain during periods of wakefulness, and prevents the buildup from reaching and activating these receptors. Without caffeine’s interference in this process, the activated receptors react by suppressing the central nervous system, a response experienced by the user as the sensation of drowsiness. In addition to delaying the onset of sleepiness, the lowered adenosine activity in caffeine’s presence encourages the release of other neurotransmitters with more stimulating effects.