All three chemicals are present in caffeinated drinks, and they work to stimulate the central nervous system, cardiac muscles, respiratory system and to delay fatigue. All these allow you to stay awake longer for those late-night cram sessions. Caffeine is an addictive drug. It operates using the same mechanisms that amphetamines, cocaine and heroin use to stimulate the brain. Yet, caffeine's effects are more mild than amphetamines, cocaine and heroin.
Caffeine occurs naturally in many plants, including coffee beans, tea leaves and cocoa nuts.
It is also found in a wide range of food products. Caffeine is added artificially to many others, including a variety of beverages. Caffeine is present in varying amounts in different beverages. Brewed coffee generally has 80 mg to 135 mg of caffeine per 12-ounce cup, while espresso tends to have 100 mg or more, depending on the strength. Sodas and teas usually contain 30 mg to 50 mg per 12-oz serving, although many sodas add more to purposefully market highly caffeinated beverages.