The main active constituent of tea and coffee is caffeine. Caffeine is a central nervous system (CNS) stimulant of the methylxanthine class of psychoactive drugs. It is the world’s most widely consumed psychoactive drug, but unlike many other psychoactive substances, it is legal and unregulated in nearly all parts of the world. Caffeine is a bitter substance found in coffee, tea, soft drinks, chocolate, kola nuts, and certain medicines. It has many effects on the body’s metabolism, including stimulating the central nervous system. This can make you more alert and give you a boost of energy.