THC — delta-9-tetrahydrocannabinol (Δ9-THC) — is the principal psychoactive constituent of Cannabis sativa, and the single compound around which most hemp law is built. It is the molecule that produces the cannabis high, and the reason a plant is classified as drug-type cannabis or as industrial hemp.
Pharmacologically, THC works mainly by activating the CB1 cannabinoid receptor in the brain — part of the body's endocannabinoid system — which underlies its effects on mood, perception, appetite and memory. Its non-intoxicating cousin CBD does not activate CB1 in the same way, which is why it is not psychoactive.
Because THC is the marker of psychoactivity, regulators use its concentration as the dividing line: a crop with delta-9 THC at or below 0.3% of dry weight counts as hemp in the United States and the European Union, while anything above is treated as a controlled drug. Most of the thresholds and tests catalogued in this Atlas — field limits, extract limits, lab methods — are ultimately about measuring this one molecule.
- NIH/NLM — Actions of delta-9-THC in cannabis (CB1 receptor, psychoactivity)
- European Commission — Hemp (0.3% delta-9 THC hemp/drug threshold)
