Human beings love their caffeine. In fact, caffeine is the most used drug in the world. For this reason, caffeine, one of the most active ingredients in coffee drinks, has been studied extensively to help us understand how it affects our bodies. Caffeine can also be found in a variety of sources such as tea, cocoa, and cola drinks.

For many individuals, coffee can one help stay awake and alert throughout the day. Caffeine is a stimulant that competes for the same protein spaces as a sleep-promoting chemical, known as adenosine. Caffeine also stimulates the heart and smooth muscles, increases stomach secretions, and promotes urination.

Research suggests that moderate amounts of caffeine affect the following behavior of adults:

• Caffeine side effects include increasing alertness and reducing fatigue, which may be significant in low arousal situations (e.g. late night work shifts).
• One of the benefits of caffeine is that it improves performance on attentiveness tasks and simple tasks that require a sustained response. These effects are often clearest with reduced alertness, although there is evidence that benefits may still occur when the person is unaffected. The effects on more challenging tasks are difficult to assess and most likely dependent on interactions between caffeine and other variables that increase alertness (e.g. personality and time of day).
• Regular caffeine consumption appears to be beneficial, with higher users having better mental functioning.
• Most individuals are experienced at controlling their caffeine consumption to maximize the above positive effects.
• Caffeine use increases the metabolic rate

Too much caffeine can yield negative side effects including the following:

• Caffeine can increase blood pressure in people who do not drink caffeinated drinks regularly. High blood pressure is directly correlated with an increase in strokes and brain disease, which increase the risk of dementia.
• Caffeine may inhibit fine motor movement control (e.g. producing shaky hands).
• It can increase the secretion of the stress hormone, cortisol.
• Caffeine makes its users have difficulty falling or staying asleep.
• It may be addictive and dependence can occur.
• Postmenopausal women may suffer from accelerated bone loss with high caffeine consumption.
• Other short-term side effects include headache, nausea, and anxiety.

A short-term overdose of caffeine is usually an excess of approximately 300 milligrams depending on body weight and level of caffeine tolerance. An overdose can result in a state of central nervous system over-stimulation, called caffeine intoxication or "caffeine jitters". The American Academy of Pediatrics recommends that adolescents consume less than 100 milligrams of caffeine a day. Younger children should not drink any caffeinated beverages on a regular basis.

The following is an example list of drinks that contain caffeine: