{Caffeine} is a {stimulant} drug. It occurs naturally in coffee beans, tea leaves, cocoa beans & cola nuts, commonly used in soft drinks, notably cola. If consumed in large quantities, or by particularly sensitive individuals, unpleasant side-effects can occur. These include agitation, tremors, {insomnia}, even irregular {heart} rhythm.
People who regularly consume large amounts of {caffeine} (e.g. 5+ cups of coffee/day) often find that their tolerance to the substance has increased. This requires an increased intake to obtain the same stimulant effect. Possible withdrawal symptoms include tiredness, headaches & irritability. These may occur through going without {caffeine} for just a few hours.
There is sometimes a strong psychological dependence too. Because of its {stimulant} effect, {caffeine} can improve short-term athletic performance &, therefore, its use is banned in sporting competitions. Caffeine is often included in various drug preparations, particularly in combination with certain painkillers.
