Marijuana is the Spanish-derived name for the dried leaves, stems, seeds, and flowering tops of the hemp plant (Cannabis sativa). For thousands of years, humans have consumed marijuana (in edible, smokeable, and vaporized forms) because of its psychoactive effects and its purported medical efficacies. Although its cultivation, distribution, and commercialization are sharply restricted or criminalized in most nations, the use of marijuana has not been eliminated. In fact, marijuana remains as the most common illicit drug in the world. See also: Brain; Hemp; Marijuana; Psychotomimetic drugs

The main psychoactive chemical component found in marijuana is delta-9-tetrahydrocannabinol (THC). In addition to THC, more than 100 related compounds, termed cannabinoids, are found in the plant. Research has indicated that some of these cannabinoids [in particular, THC and cannabidiol (CBD)] could be beneficial in the treatment of a number of illnesses or symptoms. Although marijuana has not been federally approved as a medication in the United States, the potential medical opportunities provided by this drug have led to the legal availability of medical marijuana in more than 20 states. Medical marijuana is also available in other countries, including Canada, Germany, Israel, Spain, the Netherlands, and the United Kingdom. In all of these locations, the use of medical marijuana is tightly regulated, and the conditions for its use include many restrictions (for example, the production and selling of marijuana in smokeable forms are often strictly prohibited, although nonsmokeable versions of the drug, including pills, oils, and vapors, are allowed). See also: Endocannabinoids; Pharmacology; Pharmacy

Scientific investigations have focused on the therapeutic uses of the cannabinoids found in medical marijuana to treat and ease the symptoms of various health conditions, including autoimmune diseases, various cancers, pain, inflammation, seizures (for example, epilepsy), and a number of psychiatric disorders. In the United States, THC formulations have been federally approved for the relief of nausea resulting from cancer chemotherapy and for the stimulation of appetite in AIDS patients experiencing severe weight loss. Individuals using THC medications have also reported decreased pain, inflammation, and spasticity. Similarly, CBD has shown indications of being able to reduce pain and inflammation and to limit epileptic seizures. However, before any U.S. federal approval is given for the use of medical marijuana, further research is necessary to examine the potential harmful aspects of marijuana use, including deficits related to memory, behavior, learning, and brain development. In particular, comprehensive clinical trials must show that the benefits derived from the use of the marijuana plant (as opposed to its individual cannabinoid constituents) outweigh the risks in patients using medical marijuana. See also: Analgesic; Cancer (medicine); Chemotherapy and other antineoplastic drugs; Epilepsy; Inflammation; Pain; Seizure disorders