Dose
Caffeine is used at dosages ranging from 5-200 mg. On average, a drip-brewed cup of coffee contains 100-150 mg caffeine, a can of cola contains 35-45 mg, and a cup of tea contains 40-60 mg.

Price
Caffeine is widely available both in foods and beverages as well as in inexpensive capsules and tablets.

Law
Caffeine is legal in every country around the world. In the United States, Caffeine is approved as a food additive by the Food and Drug Administration (FDA). For beverages, the FDA limits caffeine content to 72 mg per 12 ounce (355 ml) serving. It is unclear why some brands of "energy drinks" appear to be legally sold with levels of caffeine above those allowed by the FDA.

Chemistry
Caffeine is a xanthine alkaloid, with a chemical name 1,3,7-trimethylxanthine (C8H10N4O2). Its structure is very similar to the stimulants theobromine (3,7-dimethylxanthine) and theophylline (1,3-dimethylxanthine), which are frequently found in the same plants as caffeine.

Pharmacology
When orally administered, caffeine is readily absorbed by the stomach and small intestine. It has a highly-variable elimination half-life, approximately 8-9 hours in adults. It is partially metabolized by the liver into the active dimethylxanthine stimulants theobromine and theophyline, along with other metabolites. Caffeine blockades receptors of the inhibitory neurotransmitter adenosine, and this effect is believed to be the primary mechanism of its stimulant action. Caffeine also disinhibits GABA, the brain's primary inhibitory neurotransmitter.

History
Throughout caffeine's long history of use, which dates back into pre-history, it has been both praised and damned for its effects. Today caffeine is greeted with near-universal approval and caffeine may well be the most commonly-used psychoactive substance on the planet. Caffeine-producing plants like tea and coffee have been heavily cultivated and have played an important role in commerce throughout recorded history.

Onset
When taken orally, caffeine takes 15-45 minutes to reach its peak effects in the body although stimulant effects are noticeable at 5-10 minutes.

Duration
The effects of caffeine are most strongly felt for approximately 2 to 4 hours after onset.

The Experience
Caffeine causes stimulation, arousal, wakefulness, and increased alertness. It increases urine production (diuretic), causes blood vessels to contract (vassoconstrictor), and generally increases blood pressure.

Addiction Potential
Caffeine use leads to physiological dependency and tolerance. Discontinuation of regular use causes mild to moderate withdrawal symptoms, including sluggishness, fatigue, headaches, mild depression, and sometimes restlessness.

Contraindications

• People suffering from ulcers may find that caffeine worsens these problems.
• Those with generalized anxiety disorder or panic disorder should use caution in consuming caffeine, which may trigger panic attacks in sensitive populations.
• Pregnant mothers may wish to avoid caffeine. Some studies have found a correlation between moderate caffeine intake and miscarriage while other studies have found no correlation.