Cannabis, also known as marijuana among other names, is a psychoactive drug from the Cannabis plant intended for medical or recreational use. The main psychoactive part of cannabis is tetrahydrocannabinol (THC); one of 483 known compounds in the plant, including at least 65 other cannabinoids. Cannabis can be used by smoking, vaporizing, within food, or as an extract.

Cannabis is often used for its mental and physical effects, such as a "high" or "stoned" feeling, a general change in perception, euphoria (heightened mood), and an increase in appetite. Onset of effects is within minutes when smoked, and about 30 to 60 minutes when cooked and eaten. They last for between two and six hours. Short-term side effects may include a decrease in short-term memory, dry mouth, impaired motor skills, red eyes, and feelings of paranoia or anxiety. Long-term side effects may include addiction, decreased mental ability in those who started as teenagers, and behavioral problems in children whose mothers used cannabis during pregnancy. Studies have found a strong relation between cannabis use and the risk of psychosis, though the cause-and-effect relationship is debated.

Cannabis is mostly used recreationally or as a medicinal drug, although it may also be used for religious or spiritual purposes. In 2013, between 128 and 232 million people used cannabis (2.7% to 4.9% of the global population between the ages of 15 and 65). In 2015, 43% of Americans had ever used cannabis, which increased to 51% in 2016. About 12% have used it in the past year, and 7.3% have used it in the past month. This makes it the most commonly used illegal drug both in the world and the United States.

The earliest recorded uses date from the 3rd millennium BC. Since the early 20th century, cannabis has been subject to legal restrictions. The possession, use, and sale of cannabis is illegal in most countries of the world. Medical cannabis refers to the physician-recommended use of cannabis, which is taking place in Canada, Belgium, Australia, the Netherlands, Germany, Spain, and 23 U.S. states. Cannabis use started to become popular in the U.S. in the 1970s. Support for legalization has increased in the United States and several U.S. states have legalized recreational or medical use.

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