What Is THC? The High, the Medicine, and the Evidence

Scientifically reviewed by Chana Frenkel, Ph.D. · Edited by Nani Frenkel, chief editor.

Every argument about cannabis eventually becomes an argument about one molecule. THC is the reason people get high, the basis of two prescription medicines, dronabinol and nabilone, the reason so many patients reach for cannabis, and the number printed largest on many cannabis labels. For decades it was treated as a problem to be explained away. The research now tells a richer story: a molecule that works through the body’s own signaling system, with real medical uses and risks that rise with dose, frequency and individual vulnerability.

Here is what THC is, what it does in the body, what it treats according to both trials and patients, how to keep the risks in proportion, and how a lab turns a flower into the potency number on the jar.

What is THC?

THC is short for delta-9-tetrahydrocannabinol, the main intoxicating compound in cannabis. Its structure was worked out in 1964 by Yehiel Gaoni and Raphael Mechoulam at the Weizmann Institute, from Moroccan hashish, in a two-page paper in the Journal of the American Chemical Society^[1].

The “delta-9” part names where a double bond sits in the ring. Move it to position 8 and you get delta-8-THC, a related, weaker molecule; our article on THC isomers: delta-8, delta-9 and delta-10 explains why one bond matters so much. On this page, “THC” means delta-9 unless we say otherwise, which is also the convention in the pharmacology literature^[2].

The living plant actually makes very little THC. It makes THCA, an acid form that heat converts into THC, almost completely when cannabis is smoked^[2], which is why raw flower doesn’t get you high (our THCA research review covers the acid itself). Because THC is lighter than THCA, a gram of THCA can become at most 0.877 grams of THC, the factor behind every “total THC” number and written into federal hemp-testing rules^[3]; our decarboxylation explainer has the details.

How does THC act in the body?

THC works through the endocannabinoid system, a signaling network the body uses to fine-tune pain, appetite, mood, memory and more. Your nerves make their own cannabis-like messengers, the endocannabinoids, and they act on two main receptors, CB1 and CB2. THC fits both. In receptor pharmacology terms it is a partial agonist at CB1 and CB2: it switches the receptors on, but less strongly than the strongest synthetic cannabinoids do, and somewhat less strongly than the body’s own anandamide at CB1^[4]. CB1 sits densely in the brain, which is where the high, the memory lapses and the appetite come from. Our pillar on the endocannabinoid system maps the receptors.

How THC reaches those receptors matters as much as how much you take. Smoked or vaped, it peaks in the blood within about 9 minutes; eaten, it peaks 1 to 5 hours later, and only around 4% to 20% of the dose gets into the blood^[2]. On the way, the liver turns a large share of swallowed THC into 11-OH-THC, an active metabolite that reaches about the same blood levels as THC after an edible, compared with only a tenth after smoking^[2]. That slower, different mix is why edibles take longer to act, last longer and can feel more intense, and why a second dose taken too early catches so many people out; our article on 11-OH-THC tells the full story.

What is THC approved to treat?

Two THC medicines have been on the US market since the mid-1980s. Neither is a plant extract.

Dronabinol is synthetic delta-9-THC itself. Its current US label approves it in adults for two uses: loss of appetite with weight loss in people with AIDS, and chemotherapy nausea and vomiting in people who haven’t responded to conventional anti-nausea drugs^[5]. The starting dose for appetite is 2.5 mg twice a day^[5].

Nabilone is a synthetic molecule built to mimic THC. It is approved only for chemotherapy nausea and vomiting that conventional treatments haven’t controlled, and the label says it is not meant as a first-line anti-nausea drug^[6].

Both approvals are for specific patients at small measured doses, and both have been on the market for four decades.

What do people use THC for, and what does the research say?

Patients are not waiting for permission. About 27% of adults in the United States and Canada report having used cannabis for a medical reason, according to a 2026 JAMA review of studies published through September 2025^[7]. A large North American survey found the same 27%, and in it the most common reasons were pain (53%), anxiety (52%), sleep (46%), depression (40%), headaches and migraine (35%), appetite (22%), nausea (21%) and PTSD or trauma (17%)^[8].

Below is each use, with the newest trial evidence we could find and what patients report. Two things to keep in mind. Many trials test THC together with CBD, so where that is the case we say so. And patient surveys are self-reports from people who chose cannabis and stayed with it; they tell us what users experience, not what a placebo-controlled trial would find.

Chronic pain

Trials: still the best-studied use. A systematic review published in December 2025 in Annals of Internal Medicine pooled 25 placebo-controlled trials with 2,303 people, most of them with nerve pain^[9]. THC-dominant products reduced pain by about 0.8 points on a 0–10 scale, and the synthetic THC drug nabilone by about 1.6 points; balanced THC:CBD mouth sprays reduced it by about half a point. Products with little THC did not clearly help, and THC-containing products increased dizziness, sleepiness and nausea^[9].

Patients: patient surveys report much larger effects than trials, though selection, recall and expectation make the two hard to compare. Among 1,513 New England dispensary members, 76.7% of those who used opioids said they had reduced them^[10].

Chemotherapy nausea and vomiting

Trials: an approved use^{[5, 6]}, and the 2026 JAMA review found that pooled trials show cannabinoids reduce nausea and vomiting^[7]. The strongest recent evidence is a phase 2/3 trial published in 2024: in 147 patients whose nausea had persisted despite standard drugs, adding a capsule of 2.5 mg THC and 2.5 mg CBD three times a day raised complete response from 8% to 24%^[11]. The cost was more sedation (18% versus 7%), dizziness (10% versus 0%) and brief anxiety (4% versus 1%), with no serious side effects attributed to the capsules^[11].

Appetite and weight loss

Trials: dronabinol is approved for appetite loss with weight loss in AIDS^[5], and the 2026 JAMA review found a moderate effect on body weight in people with HIV/AIDS compared with placebo^[7].

Multiple sclerosis and motor neuron disease

Trials: the THC:CBD mouth sprays that reduced pain in the 2025 review^[9] were approved in Canada in 2005 as an add-on treatment for MS-related nerve pain^[12]. For MS spasticity, the older pooled evidence showed a small average improvement across seven trials^[13]. In motor neuron disease, a 6-week trial of the same type of spray in 60 patients produced a small but significant improvement in spasticity compared with placebo^[14].

Migraine

Trials: controlled research has finally arrived. In a 2026 randomized crossover trial, 92 adults treated 247 migraine attacks with vaporized flower: THC-dominant (6% THC), CBD-dominant (11% CBD), balanced (6% THC plus 11% CBD) or placebo^[15]. Two hours later, the balanced flower had relieved pain in 67% of attacks versus 47% on placebo, and stopped it completely in 35% versus 16%. THC-dominant flower relieved pain (69%) but did not beat placebo on complete pain freedom, and CBD-dominant flower did not beat placebo at all^[15]. The trial tested treatment of single attacks, not daily prevention.

Agitation in Alzheimer’s disease

Trials: this one tests THC itself. In a 2026 randomized trial, 75 people with Alzheimer’s disease and agitation took dronabinol titrated to 10 mg a day, or placebo, for three weeks^[16]. Agitation improved significantly more on dronabinol on one of the two main measures; the other moved the same way but fell short of statistical significance. Sleepiness was the only side effect that increased^[16]. Three weeks is too short to settle long-term safety, but it is a credible signal in a condition with few good treatments.

Tourette syndrome

Trials: in a 2023 crossover trial in NEJM Evidence, 22 adults with severe Tourette syndrome took an oil titrated up to 20 mg THC and 20 mg CBD a day^[17]. Tic scores fell by 8.9 points on THC:CBD versus 2.5 points on placebo, from a baseline of about 36. Side effects were mostly mild cognitive ones such as slowed thinking^[17]. A 2026 Lancet Psychiatry meta-analysis counted tic disorders among the few psychiatric conditions where THC:CBD showed benefit^[18].

Crohn’s disease

Trials: cannabis seems to help people with Crohn’s feel better, but not to heal the gut. In a 2013 study of 21 patients whose disease had not responded to steroids or biologics, 10 of 11 on THC-rich cannabis had a clinical response after 8 weeks, compared with 4 of 10 on placebo^[19]. A larger 2021 trial of 56 patients, using an oil with 160 mg/ml CBD and 40 mg/ml THC, again improved symptoms and quality of life, but endoscopy scores, CRP and calprotectin, the measures of intestinal inflammation, did not improve^[20]. The authors concluded that cannabis for Crohn’s should be used only within clinical trials^[20].

Sleep

Trials: not yet supportive. The 2026 JAMA review found that trials do not support cannabis for insomnia as a condition^[7].

Patients: 65.2% of New England dispensary members who used sleep medications said cannabis let them reduce them^[10]. Our review of THC, CBD and CBN for sleep goes deeper.

Anxiety, depression and PTSD

Trials: controlled evidence does not currently support THC as a treatment for anxiety, depression or PTSD. In a randomized trial of 80 veterans with PTSD, high-THC, high-CBD and balanced smoked cannabis all failed to beat placebo over three weeks, although every group, placebo included, improved^[21]. A 2026 Lancet Psychiatry meta-analysis of 54 randomized trials likewise found no significant benefit for anxiety or PTSD and no randomized evidence for depression^[18].

Patients: these are among the most common reasons people give for medical use^[8], and 71.8% of New England dispensary members who used anti-anxiety medications reported reducing them^[10]. This is the widest gap between what patients report and what trials have shown.

Glaucoma

A popular claim that doesn’t hold up. THC does lower eye pressure, but only for a few hours, and the American Academy of Ophthalmology does not recommend cannabis for glaucoma: the effect wears off too quickly, steady control would mean near-constant intoxication, and lower blood pressure may reduce blood flow to the optic nerve^[22].

Put together, the picture is encouraging where it counts. Good trials show real benefits for chemotherapy nausea, appetite, chronic pain and spasticity, and newer trials add acute migraine, Tourette syndrome and agitation in Alzheimer’s disease. Crohn’s patients feel better without less inflammation. For anxiety, depression and PTSD, patients report help that controlled trials have not confirmed, which is a reason for better trials, not for dismissing patients.

What are the downsides?

THC isn’t risk-free, and good information makes it safer to use.

Side effects: the most common in trials are dizziness, sleepiness and nausea^[9]. Starting low helps.

Driving: inhaled THC measurably impairs driving. In an on-road trial, lane weaving increased 40 to 100 minutes after vaping 13.75 mg of THC and was gone by 4 to 5 hours^[23]; edibles act later and longer^[2]. Don’t drive after using it, and don’t combine it with alcohol or sedatives such as benzodiazepines^[7].

Dependence: a 2024 meta-analysis found cannabis use disorder in about 25% of people using cannabis medically (29% when measured over the past 6 to 12 months), comparable to the 22% seen in recreational users; the authors call that substantial^[24]. Our piece on cannabis use disorder separates fact from fiction.

Cannabinoid hyperemesis syndrome: paradoxically, long-term heavy use can cause repeated bouts of severe nausea, vomiting and abdominal pain. Gastroenterologists define it by use more than four times a week for over a year, with symptoms that resolve after stopping^[25].

Heart and circulation: in large observational data, daily inhaled cannabis users had higher rates of coronary heart disease, heart attack and stroke than non-daily users, and the 2026 JAMA review lists ischemic heart disease among the conditions where risks likely outweigh benefits^[7]. Smoking itself and other factors make cause hard to isolate.

Pregnancy, young people and other medicines: pregnancy is a contraindication, other medicines should be checked for interactions before starting, and in adolescents high-potency cannabis has been linked to psychotic symptoms and anxiety disorders^[7].

Psychosis: the risk concentrates in a specific pattern of use. In a large European case-control study, daily use of high-potency cannabis (10% THC or more) was associated with nearly five times the odds of a psychotic disorder compared with never using; for daily use of lower-potency cannabis the figure was about 2.2 times^[26]. That is an association, not proof of cause, and most users never develop psychosis. People with a personal or family history of psychosis should talk with a clinician before using high-THC products.

Is today’s cannabis stronger?

Much stronger. An analysis of 38,681 cannabis samples seized by US authorities found average THC rose from about 4% in 1995 to about 12% in 2014^[27]. Over the same period, CBD fell, and the THC-to-CBD ratio jumped from about 14 to 1 to about 80 to 1^[27]. Those were seized, illicit samples. Our article on the push for ever-higher THC follows that trend into the legal market, and our look at THC overconsumption covers what happens when dose outruns tolerance.

Higher potency is not the same as better medicine. The approved medicines start at a few milligrams^[5], and the psychosis signal tracked high-potency daily use specifically^[26].

How do labs measure THC?

The number on a label is usually not THC alone. Labs commonly use liquid chromatography, which measures THC and THCA separately without heating the sample. The label figure is then calculated as total THC: delta-9-THC plus 0.877 times THCA^[3]. A flower at 0.5% THC and 22% THCA reports about 19.8% total THC, the maximum it could deliver if all of its THCA converted when it burns.

That calculated maximum is rarely what a user actually inhales, since combustion destroys some THC and much of the smoke never reaches the blood^[2]. Different labs can also report different numbers for the same batch. Our guide to potency testing explains the methods, and why two labs give different THC numbers explains the gap.

Is THC legal?

It depends on where you are, what product it is, and, in the US, which date it is.

Marijuana: under a federal order effective 28 April 2026, marijuana in FDA-approved drug products and marijuana handled under a state medical license moved from Schedule I to Schedule III^[28]. That did not make every medical sale federally lawful: state licensees must still register with the DEA and follow Schedule III controls^[28]. Everything else, including state-licensed adult-use cannabis, remains Schedule I. A hearing on broader rescheduling took place in summer 2026 and has not produced a decision.

Hemp: the federal definition of hemp is changing in two steps. From 12 November 2026, products containing cannabinoids the plant cannot naturally produce fall outside the definition. From 11 December 2026, the 0.3% limit applies to total THC including THCA, and finished hemp products may contain no more than 0.4 mg of total THC per container^{[29, 30]}. That cap is per package, not per serving: a single 5 mg gummy exceeds it more than twelve times over. Our explainer on the 2026 hemp redefinition sets out what counts and when.

State law varies and can be stricter or looser than federal law. None of this is legal advice.

So what is THC, really?

It is a molecule that speaks the body’s own chemical language: THC works because the endocannabinoid system was already there, regulating pain, appetite, mood and sleep, and THC plugs into the same receptors. It is an approved medicine for nausea and appetite loss; a treatment that produces small average reductions in chronic pain, particularly nerve pain; and a promising one in trials for migraine, Tourette syndrome and Alzheimer’s agitation. Patients also report help with sleep and anxiety that trials have yet to confirm. Its risks are real, increase with dose, frequency and potency, and can often be reduced, but not eliminated. The plant makes it as an acid; heat finishes the job; your liver turns some of it into something stronger.

Most of the arguments about THC fail because they ignore dose. The pharmacy starts at 2.5 mg, and plenty of patients do well at doses that small. Start low, go slow, and match the product to the purpose. For the non-intoxicating cannabinoid on the other side of most labels, read our companion piece on CBD and the dose gap between medicine and the market.

References

1. Gaoni Y, Mechoulam R. Isolation, structure, and partial synthesis of an active constituent of hashish. J Am Chem Soc. 1964;86:1646-1647.
2. Huestis MA. Human cannabinoid pharmacokinetics. Chem Biodivers. 2007;4(8):1770-1804.
3. USDA Domestic Hemp Production Program, 7 CFR §990.1 (definition of decarboxylated: delta-9 THC + 0.877 × THCA). eCFR, current as of July 2026.
4. Pertwee RG. The diverse CB1 and CB2 receptor pharmacology of three plant cannabinoids: Δ9-tetrahydrocannabinol, cannabidiol and Δ9-tetrahydrocannabivarin. Br J Pharmacol. 2008;153(2):199-215.
5. US Food and Drug Administration. Marinol (dronabinol) capsules, prescribing information. Revised March 2026.
6. Cesamet (nabilone) capsules, prescribing information. Label text as reproduced by Drugs.com, updated May 2026.
7. Hsu M, Shah A, Jordan A, et al. Therapeutic use of cannabis and cannabinoids: a review. JAMA. 2026;335(4):345-359.
8. Leung J, Chan G, Stjepanović D, et al. Prevalence and self-reported reasons of cannabis use for medical purposes in USA and Canada. Psychopharmacology. 2022;239(5):1509-1519.
9. Chou R, Fu R, Ahmed AY, et al. Cannabis-based products for chronic pain: an updated systematic review. Ann Intern Med. 2026;179(2):230-241.
10. Piper BJ, DeKeuster RM, Beals ML, et al. Substitution of medical cannabis for pharmaceutical agents for pain, anxiety, and sleep. J Psychopharmacol. 2017;31(5):569-575.
11. Grimison P, et al. Oral cannabis extract for secondary prevention of chemotherapy-induced nausea and vomiting: final results of a randomized, placebo-controlled phase II/III trial. J Clin Oncol. 2024. JCO.23.01836.
12. Patented Medicine Prices Review Board (Canada). Sativex: Notice of Compliance 15 April 2005; adjunctive treatment of neuropathic pain in adults with multiple sclerosis.
13. Whiting PF, Wolff RF, Deshpande S, et al. Cannabinoids for medical use: a systematic review and meta-analysis. JAMA. 2015;313(24):2456-2473.
14. Riva N, Mora G, Sorarù G, et al. Lancet Neurol. 2019 (CANALS trial of nabiximols for spasticity in motor neuron disease; online 13 December 2018).
15. Schuster NM, Wallace MS, Marcotte TD, et al. Vaporized cannabis versus placebo for acute migraine: a randomized, double-blind, placebo-controlled crossover trial. 2026.
16. Rosenberg PB, Amjad H, et al. A randomized controlled trial of the safety and efficacy of dronabinol for agitation in Alzheimer’s disease. Am J Geriatr Psychiatry. 2026;34(2):167-179.
17. Mosley P, et al. Tetrahydrocannabinol and cannabidiol in Tourette syndrome. NEJM Evidence. 2023.
18. Wilson J, Dobson O, Langcake A, et al. The efficacy and safety of cannabinoids for the treatment of mental disorders and substance use disorders: a systematic review and meta-analysis. Lancet Psychiatry. 2026;13:304-315.
19. Naftali T, Bar-Lev Schleider L, Dotan I, et al. Cannabis induces a clinical response in patients with Crohn’s disease: a prospective placebo-controlled study. Clin Gastroenterol Hepatol. 2013;11(10):1276-1280.e1.
20. Naftali T, Bar-Lev Schleider L, Almog S, et al. Oral CBD-rich cannabis induces clinical but not endoscopic response in patients with Crohn’s disease, a randomised controlled trial. J Crohns Colitis. 2021;15(11):1799-1806.
21. Bonn-Miller MO, Sisley S, Riggs P, et al. The short-term impact of 3 smoked cannabis preparations versus placebo on PTSD symptoms: a randomized cross-over clinical trial. PLoS One. 2021;16(3):e0246990.
22. American Academy of Ophthalmology. Why eye doctors don’t recommend marijuana for glaucoma. December 13, 2024.
23. Arkell TR, Vinckenbosch F, Kevin RC, et al. Effect of cannabidiol and Δ9-tetrahydrocannabinol on driving performance: a randomized clinical trial. JAMA. 2020;324(21):2177-2186.
24. Dawson D, Stjepanović D, Lorenzetti V, et al. The prevalence of cannabis use disorders in people who use medicinal cannabis: a systematic review and meta-analysis. Drug Alcohol Depend. 2024;257:111263.
25. American Gastroenterological Association. AGA clinical practice update on diagnosis and management of cannabinoid hyperemesis syndrome (summary). Gastroenterology.
26. Di Forti M, Quattrone D, Freeman TP, et al. The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI): a multicentre case-control study. Lancet Psychiatry. 2019;6(5):427-436.
27. ElSohly MA, Mehmedic Z, Foster S, et al. Changes in cannabis potency over the last 2 decades (1995-2014): analysis of current data in the United States. Biol Psychiatry. 2016;79(7):613-619.
28. Drug Enforcement Administration. Schedules of controlled substances: rescheduling of FDA-approved products containing marijuana and of marijuana subject to a state medical marijuana license; final order. Federal Register document 2026-08176, 91 Fed. Reg. 22714 (Apr. 28, 2026); effective April 28, 2026.
29. Continuing Appropriations, Agriculture, Legislative Branch, Military Construction and Veterans Affairs, and Extensions Act, 2026, P.L. 119-37, Div. B, §781 (Nov. 12, 2025), amending 7 U.S.C. §1639o. govinfo.
30. Continuing Appropriations and Extensions Act, 2027, H.R. 6500, §2019 (signed Sept. 2, 2026), delaying most of the new hemp definition to Dec. 11, 2026. govinfo.

This article is an educational summary of published research and regulation. It is not medical or legal advice.

Last updated: November 2026.