THC, referred to as dronabinol in the pharmaceutical context, is approved in the United States as a capsule or solution to relieve chemotherapy-induced nausea and vomiting and HIV/AIDS-induced anorexia.

[11] THC is an active ingredient in nabiximols, a specific extract of Cannabis that was approved as a botanical drug in the United Kingdom in 2010 as a mouth spray for people with multiple sclerosis to alleviate neuropathic pain, spasticity, overactive bladder, and other symptoms.

[15] Side effects of THC include red eyes, dry mouth, drowsiness, short-term memory impairment, difficulty concentrating, ataxia, increased appetite, anxiety, paranoia, psychosis (i.e., hallucinations, delusions), decreased motivation, and time dilation, among others.

[16][17] Chronic usage of THC may result in cannabinoid hyperemesis syndrome (CHS), a condition characterized by cyclic nausea, vomiting, and abdominal pain that may persist for months to years after discontinuation.

[25] The psychoactive effects of THC are primarily mediated by the activation of (mostly G-coupled) cannabinoid receptors, which result in a decrease in the concentration of the second messenger molecule cAMP through inhibition of adenylate cyclase.

[citation needed] THC is a lipophilic molecule[27] and may bind non-specifically to a variety of entities in the brain and body, such as adipose tissue (fat).

[28][29] THC, as well as other cannabinoids that contain a phenol group, possess mild antioxidant activity sufficient to protect neurons against oxidative stress, such as that produced by glutamate-induced excitotoxicity.

However, clinical trials need to be performed in order to assess the importance of ATX inhibition by THC during medicinal cannabis consumption.

[22][3] THC is rapidly distributed into well-vascularized organs such as lung, heart, brain, and liver, and is subsequently equilibrated into less vascularized tissue.

[22] Pharmacogenomic research has found that oral THC exposure is 2- to 3-fold greater in people with genetic variants associated with reduced CYP2C9 function.

[8] As a phytochemical, THC is assumed to be involved in the plant's evolutionary adaptation against insect predation, ultraviolet light, and environmental stress.

[39][40][41] The preparation of THC was reported in 1965. that procedure called for the intramolecular alkyl lithium attack on a starting carbonyl to form the fused rings, and a tosyl chloride mediated formation of the ether.

[46] Cannabidiol was isolated and identified from Cannabis sativa in 1940 by Roger Adams who was also the first to document the synthesis of THC (both Delta-9-THC and Delta-8-THC) from the acid-based cyclization of CBD in 1942.

[57] THC and its 11-OH-THC and THC-COOH metabolites can be detected and quantified in blood, urine, hair, oral fluid or sweat using a combination of immunoassay and chromatographic techniques as part of a drug use testing program or in a forensic investigation.

[61][62] THC, along with its double bond isomers and their stereoisomers,[63] is one of only three cannabinoids scheduled by the UN Convention on Psychotropic Substances (the other two are dimethylheptylpyran and parahexyl).

[66] In 2003, the World Health Organization Expert Committee on Drug Dependence recommended transferring THC to Schedule IV of the convention, citing its medical uses and low abuse and addiction potential.

[68] As of 2023, 38 states, four territories, and the District of Columbia in the United States allow medical use of cannabis (in which THC is the primary psychoactive component), with the exception of Georgia, Idaho, Indiana, Iowa, Kansas, Nebraska, North Carolina, South Carolina, Tennessee, Texas, Wisconsin, and Wyoming.

[74] A 2015 review confirmed that medical marijuana was effective for treating spasticity and chronic pain, but caused numerous short-lasting adverse events, such as dizziness.

[75] Preliminary research indicates that prolonged exposure to high doses of THC may interfere with chromosomal stability, which may be hereditary as a factor affecting cell instability and cancer risk.

The carcinogenicity of THC in the studied populations of so-called "heavy users" remains dubious due to various confounding variables, most significantly concurrent tobacco use.