Caffeine () is a central nervous system (CNS) stimulant. 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. It is a bitter, white crystalline purine, a methylxanthine alkaloid, and thus closely related chemically to the adenine and guanine contained in deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). It is found in the seeds, nuts, or leaves of a small number of plants native to South America. The most well known source of caffeine is the seed (commonly incorrectly referred to as the “bean”) of the Coffea arabica coffee plant. Beverages containing caffeine are ingested to relieve or prevent drowsiness and to increase one’s energy level. Caffeine is extracted from the plant part containing it for making beverages by steeping it in water, a process called infusion. These beverages are very popular: in North America, 90% of adults consume caffeine daily. Part of the reason caffeine is classified by the Food and Drug Administration as “generally recognized as safe” (GRAS) is that toxic doses, over 10 grams per day for an adult, are much higher than the typically used doses of under 500 milligrams per day: an over twentyfold difference. A cup of coffee contains 80–175 mg. of caffeine, depending on what “bean” (seed) is used and how it is prepared: by drip, percolation, or espresso. There are several known mechanisms of action to explain the effects of caffeine. The most prominent is to reversibly block the action of adenosine on its receptor, which blocks the onset of drowsiness induced by adenosine. Caffeine also stimulates selected portions of the autonomic nervous system. Caffeine can have both positive and negative health effects. It can be used to treat bronchopulmonary dysplasia of prematurity, and to prevent apnea of prematurity: caffeine citrate was placed on the WHO Model List of Essential Medicines in 2007. It may confer a modest protective effect against some diseases, including Parkinson’s disease and certain types of cancer. One meta-analysis concluded that cardiovascular disease such as coronary artery disease and stroke is less likely with 3–5 cups of 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. Mild physical dependence can occur with chronic caffeine use and is associated with withdrawal symptoms such as headaches and irritability. 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 confers a survival advantage on the plant containing it in three ways. First, if it is ingested by an insect feeding on and potentially damaging or killing the plant, caffeine functions as a natural pesticide which can paralyze and kill the insect. Second, droppings from the plant infuse the surrounding soil with caffeine, which can inhibit the growth of and kill competing seedlings (and potentially its own progeny and itself). Third, caffeine can enhance the reward memory of pollinators such as honey bees, thus increasing the numbers of its progeny.