Tolerance to the autonomic effects of increased blood pressure and heart rate, and increased urine output, develops with chronic use (i.e., these symptoms become less pronounced or do not occur following consistent use).
Caffeine is classified by the Food and Drug Administration as "generally recognized as safe" (GRAS).
Toxic doses, over 10 grams per day for an adult, are much higher than typical doses of under 500 milligrams per day.
Beverages containing caffeine are ingested to relieve or prevent drowsiness and to improve performance.
To make these drinks, caffeine is extracted by steeping the plant product in water, a process called infusion.
Caffeine-containing drinks, such as coffee, tea, and cola, are very popular; in 2005, 90% of North American adults consumed caffeine daily.
Caffeine can have both positive and negative health effects.
It can be used to prevent and treat bronchopulmonary dysplasia of prematurity, and to treat apnea of prematurity: caffeine citrate was placed on the WHO Model List of Essential Medicines in 2007. One meta-analysis concluded that cardiovascular disease such as coronary artery disease and stroke is less likely with 3–5 cups of non-decaffeinated coffee per day but more likely with over 5 cups per day.
Some people experience insomnia or sleep disruption if they consume caffeine, especially during the evening hours, but others show little disturbance.
Evidence of a risk during pregnancy is equivocal; some authorities recommend that pregnant women limit consumption to the equivalent of two cups of coffee per day or less.
Caffeine can produce a mild form of drug dependence – associated with withdrawal symptoms such as sleepiness, headache, and irritability – when an individual stops using caffeine after repeated daily intake.
It is the world's most widely consumed psychoactive drug.
Unlike many other psychoactive substances, it is legal and unregulated in nearly all parts of the world.
There are several known mechanisms of action to explain the effects of caffeine.