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Delta-9-tetrahydrocannabinol (?9-THC), better known to cannabis users simply as THC, is the marijuana plant's primary component for causing psychoactive effects. THC was first discovered and isolated by Bulgarian-born chemist Raphael Mechoulam in Israel in 1964. It was found that, when smoked, tetrahydrocannabinol is absorbed into the bloodstream and travels to the brain, attaching itself to the naturally-occurring endocannabinoid receptors located in the cerebral cortex, cerebellum, and basal ganglia. These are the parts of the brain responsible for thinking, memory, pleasure, coordination and movement.
THC targets receptors in a manner far less selective than endocannabinoid molecules released during retrograde signaling, as the drug has a relatively low cannabinoid receptor affinity. THC is also limited in its efficacy compared to other cannabinoids due to its partial agonistic activity, as THC appears to result in greater downregulation of cannabinoid receptors than endocannabinoids. Furthermore, in populations of low cannabinoid receptor density, THC may even act to antagonize endogenous agonists that possess greater receptor efficacy. However while THC's pharmacodynamic tolerance may limit the maximal effects of certain drugs, evidence suggests that this tolerance mitigates undesirable effects, thus enhancing the drug's therapeutic window.
The median lethal dose of THC in humans is not known. A 1972 study gave up to 9000 mg/kg of THC to dogs and monkeys without any lethal effects. Some rats died within 72 hours after a dose of up to 3600 mg/kg.
Detection in body fluids
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.
Detection in breath
Recreational use of cannabis is legal in many parts of North America, increasing the demand for THC monitoring methods in both personal and law enforcement uses. Breath sampling as a noninvasive method is in development to detect THC, which is difficult to quantify in breath samples. Scientists and industry are commercializing various types of breath analyzers to monitor THC in breath.
At its 33rd meeting, 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. In 2018 the federal farm bill was passed allowing any hemp derived product not exceeding 0.3% ?-9 THC to be sold legally. Since the law counted only ?-9 THC, ?-8 THC was considered legal to sell under the farm bill and was sold online. After August 21st, 2020, all forms of THC were deemed illegal above 0.3% under the CSA (Controlled Substances Act), according to the DEA. The ruling is currently being debated[by whom?] and companies that formerly sold forms of THC are lobbying to keep other forms of THC (other than delta-9) legal for commerce. 
The status of THC as an illegal drug in most countries imposes restrictions on research material supply and funding, such as in the United States where the National Institute on Drug Abuse and Drug Enforcement Administration continue to control the sole federally-legal source of cannabis for researchers. Despite an August 2016 announcement that licenses would be provided to growers for supplies of medical marijuana, no such licenses were ever issued, despite dozens of applications. Although cannabis is legalized for medical uses in more than half of the states of the United States, no products have been approved for federal commerce by the Food and Drug Administration, a status that limits cultivation, manufacture, distribution, clinical research, and therapeutic applications.
In April 2014, the American Academy of Neurology found evidence supporting the effectiveness of the cannabis extracts in treating certain symptoms of multiple sclerosis and pain, but there was insufficient evidence to determine effectiveness for treating several other neurological diseases. 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.
Multiple sclerosis symptoms
Spasticity. Based on the results of 3 high quality trials and 5 of lower quality, oral cannabis extract was rated as effective, and THC as probably effective, for improving people's subjective experience of spasticity. Oral cannabis extract and THC both were rated as possibly effective for improving objective measures of spasticity.
Centrally mediated pain and painful spasms. Based on the results of 4 high quality trials and 4 low quality trials, oral cannabis extract was rated as effective, and THC as probably effective in treating central pain and painful spasms.
Bladder dysfunction. Based on a single high quality study, oral cannabis extract and THC were rated as probably ineffective for controlling bladder complaints in multiple sclerosis
Huntington disease. No reliable conclusions could be drawn regarding the effectiveness of THC or oral cannabis extract in treating the symptoms of Huntington disease as the available trials were too small to reliably detect any difference
Parkinson's disease. Based on a single study, oral CBD extract was rated probably ineffective in treating levodopa-induced dyskinesia in Parkinson's disease.
Alzheimer's disease. A 2009 Cochrane Review found insufficient evidence to conclude whether cannabis products have any utility in the treatment of Alzheimer's disease.
Other neurological disorders
Tourette syndrome. The available data was determined to be insufficient to allow reliable conclusions to be drawn regarding the effectiveness of oral cannabis extract or THC in controlling tics.
Cervical dystonia. Insufficient data was available to assess the effectiveness of oral cannabis extract of THC in treating cervical dystonia.
Potential for toxicity
Preliminary research indicates that exposure to high doses of THC may lead to DNA damage (chromothripsis), which may be hereditary as a factor affecting cell instability and cancer risk.
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