Caffeine is a central nervous system (CNS) stimulant of the methylxanthine class. 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. The most prominent is that it reversibly blocks the action of adenosine on its receptor and consequently prevents the onset of drowsiness induced by adenosine. Caffeine also stimulates certain portions of the autonomic nervous system.

Caffeine 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, nuts, or leaves of a number of plants native to South America and East Asia and helps to protect them against predator insects and prevent germination of nearby seeds. The most well known source of caffeine is the coffee bean, a misnomer for the seed of Coffea plants. 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 is a central nervous system stimulant and is used to reduce physical fatigue and to prevent or treat drowsiness. It produces increased wakefulness, increased focus, and better general body coordination. The amount of caffeine needed to produce these effects varies from person to person, depending on body size and degree of tolerance. Desired effects begin approximately one hour after consumption, and a moderate dose usually subsides in about three or four hours. Caffeine can delay or prevent sleep, and improves task performance during sleep deprivation. Shift workers have fewer mistakes caused by drowsiness. At normal doses, caffeine has variable effects on learning and memory, but it generally improves reaction time, arousal, and concentration. A 2014 systematic review and meta-analysis found that concurrent caffeine and L-theanine use has synergistic psychoactive effects that promote alertness, attention, and task switching; these effects are most pronounced during the first hour post-dose.

Both caffeine and coffee are proven ergogenic aids in humans. Caffeine improves athletic performance in aerobic (especially endurance sports) and anaerobic conditions. Moderate doses of caffeine (around 5 mg/kg ) can improve sprint performance, cycling and running time trial performance, endurance (i.e., it delays the onset of muscle fatigue and central fatigue), and cycling power output

Minor undesired symptoms from caffeine ingestion not sufficiently severe to warrant a psychiatric diagnosis are common, and include mild anxiety, jitteriness, insomnia, increased sleep latency, and reduced coordination. Caffeine can have negative effects on anxiety disorders. According to a 2011 literature review, caffeine use is positively associated with anxiety and panic disorders. At high doses, typically greater than 300 mg, caffeine can both cause and worsen anxiety. For some people, discontinuing caffeine use can significantly reduce anxiety.

Low doses of caffeine cause increased alertness and decreased fatigue. In moderate doses, caffeine may reduce symptoms of depression and lower suicide risk

Withdrawal can cause mild to clinically significant distress or impairment in daily functioning. The frequency at which this occurs is self reported at 11%, but in lab tests only half of the people who report withdrawal actually experience it, casting doubt on many claims of dependence. Mild to increasingly severe physical dependence and withdrawal symptoms may occur upon abstinence, with greater than 100 mg caffeine per day; some symptoms associated with psychological dependence may also occur during withdrawal. Caffeine dependence can involve withdrawal symptoms such as fatigue, headache, irritability, depressed mood, reduced contentedness, inability to concentrate, sleepiness or drowsiness, stomach pain, and joint pain. Withdrawal headaches are experienced by roughly half of those who stop consuming caffeine for two days following an average daily intake of 235 mg.

The ICD-10 includes a diagnostic model for caffeine dependence, but the DSM-5 does not. The APA, which published the DSM-5, acknowledged that there was sufficient evidence in order to create a diagnostic model of caffeine dependence for the DSM-5, but they noted that the clinical significance of this disorder is unclear. The DSM-5 instead lists "caffeine use disorder" in the emerging models section of the manual