Cannabis Pharmacology: How THC and CBD Actually Work
A deep dive into receptor binding, signal transduction, and metabolism. For readers who want to understand the cellular mechanisms.
Cannabis produces its effects through one of the body's oldest cell-signaling systems: the endocannabinoid system (ECS). Rather than acting indiscriminately, THC and CBD engage specific receptors, enzymes, and ion channels with different affinities and consequences. Understanding the pharmacology means following what happens at the molecular level — which receptor binds, which intracellular signal changes, and how the body clears the drug. This article is a mechanistic walkthrough for readers who want the cellular "why" behind the effects.
The endocannabinoid system, briefly
The ECS has three core parts: endogenous ligands (the endocannabinoids anandamide and 2-AG), cannabinoid receptors (principally CB1 and CB2), and the enzymes that synthesize and degrade those ligands. Endocannabinoids are produced on demand from membrane lipid precursors and typically travel backward across the synapse — a process called retrograde signaling — where they act as a brake on whatever neurotransmitter was just released. Plant cannabinoids (phytocannabinoids) co-opt this machinery because their shapes resemble the body's own messengers.
CB1 receptors are among the most abundant G-protein-coupled receptors (GPCRs) in the central nervous system, densely expressed in the basal ganglia, hippocampus, cerebellum, and cortex. This distribution explains the classic effects: memory disruption (hippocampus), motor and coordination changes (cerebellum/basal ganglia), appetite stimulation (hypothalamus), and the intoxicating "high." CB2 receptors sit largely on immune cells — microglia, macrophages, and splenic tissue — and mediate anti-inflammatory and immunomodulatory effects with little influence on mood or cognition.
THC: a partial agonist at CB1 and CB2
Δ⁹-tetrahydrocannabinol is a partial agonist at both CB1 and CB2. "Partial" matters: it activates the receptor but cannot push the response to the ceiling achieved by a full agonist like the synthetic compound CP-55,940. This partial-agonist character has real pharmacological consequences — it is part of why THC has a relatively wide therapeutic window and why pure THC overdoses are rarely fatal on their own, even at doses that produce severe dysphoria.
When THC binds CB1, the receptor couples to an inhibitory G-protein (Gi/o). The α-subunit dissociates and suppresses adenylyl cyclase, lowering intracellular cAMP. The βγ-subunit does the more behaviorally interesting work: it directly inhibits voltage-gated calcium channels at the presynaptic terminal and opens inward-rectifying potassium channels. The net effect is less neurotransmitter release — dampening GABA, glutamate, dopamine, serotonin, and norepinephrine release depending on the synapse. The psychoactive experience is less "THC turns something on" and more "THC removes a presynaptic brake on a brake," which in GABAergic circuits translates to disinhibition — and the downstream dopamine release in the mesolimbic pathway that underlies reward.
CBD: pharmacologically promiscuous
Cannabidiol resists a tidy one-receptor story. CBD has low affinity for the orthosteric binding site of both CB1 and CB2, and it does not produce intoxication. Instead, its effects appear to come from a broad polypharmacology. It acts as a negative allosteric modulator of CB1 — altering the receptor's shape so that THC binds less effectively and signals less, which may blunt THC's psychotropic intensity. CBD also inhibits FAAH (fatty acid amide hydrolase), the enzyme that breaks down anandamide, effectively raising anandamide levels — a plausible mechanism for its anxiolytic and anti-seizure observations.
Beyond the classical receptors, CBD interacts with TRPV1 (the "capsaicin receptor" involved in pain and heat sensing), the 5-HT1A serotonin receptor (linked to anti-anxiety effects), and GPR55 (where it acts as an antagonist, potentially relevant to bone density and cancer cell signaling). CBD is also a low-affinity agonist at PPARγ nuclear receptors, which regulate gene expression around lipid metabolism and inflammation. This multi-target profile explains why CBD is being investigated for such a wide range of conditions — and why its effects are dose-dependent and sometimes inconsistent.
Receptor distribution and downstream effects
Where the receptors are determines what the cannabinoids do. The table below maps region to outcome.
| Region / cell type | Receptor | Effect of activation | |---|---|---| | Hippocampus | CB1 | Impaired short-term memory, reduced consolidation | | Basal ganglia | CB1 | Altered motor control, appetite, reward | | Cerebellum | CB1 | Ataxia, slowed reaction time | | Hypothalamus | CB1 | Increased appetite ("the munchies") | | Spinal cord | CB1, CB2 | Modulation of nociceptive (pain) signaling | | Immune cells | CB2 | Reduced cytokine release, anti-inflammatory |
Because CB1 is largely absent from the brainstem respiratory centers, cannabinoids do not suppress breathing the way opioids do — a key reason overdose lethality is low.
Metabolism and the cytochrome P450 system
THC is heavily metabolized in the liver, primarily by CYP2C9 and CYP3A4. The major active metabolite is 11-hydroxy-THC (11-OH-THC), which is more potent at CB1 than the parent compound and is a major contributor to the intensity of oral dosing (see our bioavailability article). Both THC and 11-OH-THC are further oxidized to the inactive THC-COOH, which is the analyte most drug tests look for. CBD, meanwhile, is metabolized by CYP3A4 and CYP2C19.
Both compounds are also potent CYP inhibitors in vitro. CBD in particular inhibits CYP2C19, CYP3A4, and CYP2C9, which is why Epidiolex (pharmaceutical CBD) carries drug-interaction warnings. Clinically relevant interactions can occur with warfarin (increased INR), clobazam (elevated active metabolite), and some statins and antidepressants. The enzyme UGT1A9 also glucuronidates cannabinoids for biliary and renal excretion. Because of genetic variation in these enzymes and tolerance-driven changes in receptor sensitivity, two people taking the same dose can have very different experiences.
Key takeaways
- THC is a partial agonist at CB1/CB2, signaling through Gi/o proteins that lower cAMP and reduce neurotransmitter release.
- CB1 is CNS-dominant (memory, motor, appetite, reward); CB2 is immune-dominant (inflammation, analgesia).
- CBD has low CB affinity but a broad polypharmacology: negative allosteric modulation of CB1, FAAH inhibition (raising anandamide), and activity at TRPV1, 5-HT1A, and GPR55.
- Metabolism runs through CYP2C9/3A4, producing the potent 11-OH-THC and then the inactive THC-COOH found on drug tests.
- Real drug interactions exist — especially with CBD and warfarin, clobazam, and CYP-metabolized psychiatric drugs.