Marijuana policies in several states in the United States have legalized marijuana for medical and/or recreational use. Marijuana refers to the dried leaves, flowers, stems, and seeds from the hemp plant, Cannabis sativa. Delta-9-tetrahydrocannabinol (THC) is the main psychoactive chemical, responsible for most of the intoxicating effects. Stronger forms of marijuana include sinsemilla (from specially tended female plants) and concentrated resins, including honey-like hash oil, waxy budder, and hard amberlike shatter.

When smoked, marijuana is quickly absorbed from the lungs into the blood and then sequestered in tissues and metabolized by the liver.

Can also be baked into foods (edibles) and eaten with a resulting slower onset of action of 30–60 min.

Cannabinoid receptors (CB₁ and CB₂) have been identified in the CNS (cerebral cortex, basal ganglia, and hippocampus) and PNS, as well as on T and B lymphocytes.

Cannabinoid effects occur in the limbic system, impacting the reward pathway and areas of pain perception. Effects include altered senses, altered sense of time, laughter, changes in mood, psychomotor retardation, difficulty with thinking and problem-solving, and impaired memory.

Marijuana is the most commonly used illicit drug in the United States, with 22.2 million (8.3%) current marijuana users aged ≥12 (i.e., users in the past 30 days). ER visits involving marijuana have increased, which may be due to increased THC levels resulting in a greater chance of a harmful reaction.

Acute intoxication brings a perceived sense of relaxation and mild euphoria, accompanied by impairment in memory, concentration, judgement, and perceptual and psychomotor function, as well as anxiety, paranoia, and rarely, psychosis. Physical signs of marijuana use include conjunctival injection and tachycardia. Adverse effects include respiratory problems due to inhaled pulmonary irritants and lower birth weights in pregnancy.

Chronic marijuana use may also have adverse psychological effects, which may not be permanent, such as impaired concentration and learning, insomnia, and worsening symptoms in schizophrenia. Upon cessation, or cutting back, there is evidence of a withdrawal syndrome consisting of irritability, insomnia, anorexia, anxiety, and craving. Individuals who begin marijuana use at age <17 may be more prone to cognitive deficits, and may be at higher risk for future polydrug addiction.

No approved medications exist for marijuana use disorder. Behavioral therapies and symptomatic treatment of withdrawal, for example selective serotonin reuptake inhibitors (SSRIs) to treat anxiety, may be effective.

Therapeutic use of marijuana includes as an antiemetic in chemotherapy, appetite promotor in AIDS, intraocular pressure reducer in glaucoma, and spasticity reducer in multiple sclerosis and other neurologic disorders.