<unk>  is a central nervous system (CNS) stimulant of the methylxanthine class. There are several known mechanisms of action to explain the effects of<unk> . The most prominent is that it reversibly blocks the action of adenosine on its receptors and consequently prevents the onset of drowsiness induced by adenosine.<unk>  also stimulates certain portions of the autonomic nervous system.<unk>  is a bitter, white crystalline purine, a methylxanthine alkaloid, and is chemically related to the adenine and guanine bases of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). It is found in the seeds, fruits, nuts, or leaves of a number of plants native to Africa, East Asia and South America, and helps to protect them against herbivores and from competition by preventing the germination of nearby seeds, as well as encouraging consumption by select animals such as honey bees. The best-known source of<unk>  is the coffee bean, the seed of the Coffea plant. People may drink beverages containing<unk>  to relieve or prevent drowsiness and to improve cognitive performance. To make these drinks,<unk>  is extracted by steeping the plant product in water, a process<unk> lled infusion.<unk> -containing drinks, such as coffee, tea, and cola, are consumed globally in high volumes. In 2020, almost 10 million tonnes of coffee beans were consumed globally.<unk>  is the world's most widely consumed psychoactive drug. Unlike most other psychoactive substances,<unk>  remains largely unregulated and legal in nearly all parts of the world.<unk>  is also an outlier as its use is seen as socially acceptable in most cultures and even encouraged in others, particularly in the Western world.<unk> <unk> n have both positive and negative health effects. It<unk> n treat and prevent the premature infant breathing disorders bronchopulmonary dysplasia of prematurity and apnea of prematurity.<unk>  citrate is on the WHO Model List of Essential Medicines. It may confer a modest protective effect against some diseases, including Parkinson's disease. Some people experience sleep disruption or anxiety if they consume<unk> , but others show little disturbance. Evidence of a risk during pregnancy is equivocal; some authorities recommend that pregnant women limit<unk>  to the equivalent of two cups of coffee per day or less.<unk> <unk> n produce a mild form of drug dependence – associated with withdrawal symptoms such as sleepiness, headache, and irritability – when an individual stops using<unk>  after repeated daily intake. 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).<unk>  is classified by the US Food and Drug Administration as generally recognized as safe. Toxic doses, over 10 grams per day for an adult, are much higher than the typical dose of under 500 milligrams per day. The European Food Safety Authority reported that up to 400 mg of<unk>  per day (around 5.7 mg/kg of body mass per day) does not raise safety concerns for non-pregnant adults, while intakes up to 200 mg per day for pregnant and lactating women do not raise safety concerns for the fetus or the breast-fed infants. A cup of coffee contains 80–175 mg of<unk> , depending on what "bean" (seed) is used, how it is roasted (darker roasts have less<unk> , and how it is prepared (e.g., drip, percolation, or espresso). Thus it requires roughly 50–100 ordinary cups of coffee to reach the toxic dose. However, pure powdered<unk> , which is available as a dietary supplement,<unk> n be lethal in tablespoon-sized amounts.
Caffeine
Methacholine
Betahistine
Nicotine
Phenylpropanolamine
Molsidomine
Piperoxan
Codeine