alkaloids , has marked physiological effects.
It occurs in {{link=coffee">coffee beans, tea leaves, kola nut s, cacao , maté, and guarana and in the products made from them. Its stimulating effect on the central nervous system , heart , cardiovascular system , and kidney s makes it medically useful in treating respiratory depression caused by overdose of barbiturate s, morphine , or heroin . Its positive effects can include improved motor performance, decreased fatigue, increased alertness, and enhanced sensory activity. Excessive caffeine can produce irritability, anxiety, insomnia, and potentially serious symptoms such as heart irregularities and delirium. Much of the caffeine included in many over-the-counter stimulants, cold remedies, and painkillers has been extracted during production of decaffeinated coffee and tea.