THC (Δ9-THC): general

THC (delta-9-tetrahydrocannabinol) is the main psychoactive compound in cannabis, one of 113 cannabinoids the plant produces [1][2]. The main psychoactive component of cannabis is (-)-trans-Δ9-THC [3]; dronabinol is the name used for that same molecule made synthetically [4]. Other stereoisomers exist, including (+)-trans, (-)-cis, and (+)-cis [5].

THC acts as a partial agonist at CB1 and CB2 cannabinoid receptors [6]. CB1 receptors, in the brain, produce the high; CB2 receptors, in the immune system, are the basis for THC's medical uses [2]. Those uses include easing chemotherapy-induced nausea and vomiting, stimulating appetite, and treating anorexia related to AIDS [1][4][6], along with approved use for chronic pain and multiple sclerosis [3]. It is also being studied for anorexia nervosa, agitation in dementia, and Tourette's syndrome [3].

What a given amount does to you depends on the product's strength, how much is taken, whether other cannabinoids are present, the setting, and your own experience with it [7]. When people stop using THC because of a bad experience, the reason is usually dysphoria, hallucination, or paranoia [1].

Two things worth knowing before anything else. First, as with cannabinoids generally, clinically relevant interactions exist with other drugs broken down by the same liver enzymes, including warfarin and valproate [8] — worth mentioning to a prescriber if you're on either. Second, synthetic cannabinoid receptor agonists, as a class, are linked to higher, less predictable toxicity than natural cannabinoids [9], and semi-synthetic products like HHC or THC-O carry risks that remain largely unknown, with potency and impurity that can differ significantly from natural THC [10]. Whatever you make of those differences, know which product you actually have in hand.

References

1. Tetrahydrocannabinol (THC) - StatPearls - NCBI Bookshelf — NCBI Bookshelf
2. Δ9-Tetrahydrocannabinol — American Chemical Society
3. Dark Classics in Chemical Neuroscience: Δ9-Tetrahydrocannabinol — ACS Chemical Neuroscience
4. Δ9-tetrahydrocannabinol | Ligand page | IUPHAR/BPS Guide to PHARMACOLOGY — IUPHAR/BPS Guide to Pharmacology
5. Delta-9-tetrahydrocannabinol - Expert Committee on Drug Dependence Information Repository — WHO Expert Committee on Drug Dependence
6. Tetrahydrocannabinol — Wikipedia
7. Tetrahydrocannabinol — Encyclopaedia Britannica
8. Systematic review of drug-drug interactions of delta-9-tetrahydrocannabinol, cannabidiol, and Cannabis — Frontiers in Pharmacology
9. ACMD Synthetic cannabinoid receptor agonists (SCRA) report — UK Advisory Council on the Misuse of Drugs
10. ‘Semi-synthetic’ Cannabinoids: Cannabinoids related to tetrahydrocannabinol and cannabidiol — UK Advisory Council on the Misuse of Drugs