What is the difference between THC and CBD?
Almost every question people bring to a consultation turns on the difference between these two compounds — which one causes the effects they are worried about, which one is in the products they have read about, and which one the evidence actually supports.
The short answer
- THC is intoxicating. It is what produces the effects associated with cannabis.
- CBD is not intoxicating, and does not produce those effects at any dose.
- They act on the body differently, which is why they have different uses and different risks.
- Most prescribed products contain both, in proportions decided by the specialist.
- Almost all the serious risks in this field come from THC, not from CBD.
What they are
Both are cannabinoids: naturally occurring compounds found in the cannabis plant. Around a hundred have been identified. These two are simply the ones that have been studied most.
Tetrahydrocannabinol, usually shortened to THC, is the compound responsible for the intoxicating effects of cannabis. Cannabidiol, usually shortened to CBD, is not intoxicating — it does not produce a high however much of it is taken.
That single difference drives almost everything else: how each is regulated, what each is used for, what side effects each causes, and how carefully a specialist approaches them.
How they act on the body
THC binds directly to cannabinoid receptors in the brain and body. CBD largely does not — its effects appear to come through other routes, which are less well understood.
THC acts as a partial agonist at the CB1 receptor, which is abundant in the brain. That direct action explains both its psychoactive effects and its unwanted ones: altered perception, impaired short-term memory, slowed reaction time, anxiety in some people, and increased heart rate.
CBD has low affinity for those receptors. Its mechanisms are still being worked out and appear to involve several different targets. This is a genuine gap in knowledge rather than a detail being glossed over — a widely cited review described CBD's pharmacology as involving a notably wide range of molecular targets.
The receptor system they act on is described in What is the endocannabinoid system?
What each is actually supported for
The strongest evidence in this field attaches to specific licensed products for specific conditions. Outside those, the picture is much less settled for both compounds.
| THC | CBD | |
|---|---|---|
| Licensed UK use | A synthetic form is licensed for chemotherapy-induced nausea and vomiting; a THC-and-CBD spray is licensed for MS spasticity | A purified form is licensed for two rare severe childhood epilepsies |
| Intoxicating | Yes | No |
| Main risks | Impairment, anxiety, psychosis risk, dependence, cardiovascular effects | Drug interactions, liver enzyme changes, drowsiness, diarrhoea |
| Driving | Subject to a specified blood limit; impairment is an offence | Not subject to the specified limit, but impairment rules still apply |
Note the asymmetry. Nearly all the caution in a specialist assessment — the questions about psychosis, about family history, about driving and safety-critical work — exists because of THC.
Why most prescriptions contain both
Products used in UK practice usually contain a mixture, and the balance between the two is one of the things a specialist adjusts.
The reasoning is partly that the two appear to modify each other's effects, and partly practical: the aim is usually to get a therapeutic effect at the lowest exposure that achieves it. How that balance is arrived at in practice is covered in How is a dose decided and adjusted?
We do not publish product names, proportions or strengths anywhere on this site. That is a legal requirement rather than an editorial choice, and the reasons are set out in Is medical cannabis legal in the UK?
Frequently asked questions
What is the difference between THC and CBD?
THC is intoxicating and acts directly on cannabinoid receptors in the brain. CBD is not intoxicating and acts through other, less well understood routes. Nearly all the serious risks associated with cannabis-based medicines come from THC.
Does CBD get you high?
No. CBD is not intoxicating at any dose. That is the clearest single difference between the two compounds.
Which one is in prescribed medicines?
Usually both, in proportions decided by the prescribing specialist. Some licensed products contain predominantly one or the other. We do not publish product names or strengths, which UK medicines advertising law does not permit.
Is CBD safe?
It is far less risky than THC, but not risk-free. It interacts with several other medicines, can affect liver enzymes, and commonly causes drowsiness and diarrhoea at medicinal doses. It should be discussed with a prescriber like any other medicine.
Sources and further reading
Every factual statement on this page is drawn from the following public sources. Links open in a new tab and are not affiliated with Flora House.
- National Academies of Sciences, Engineering, and Medicine. The Health Effects of Cannabis and Cannabinoids: The Current State of Evidence and Recommendations for Research (2017)
- British National Formulary: Cannabis extracts
- NICE guideline NG144: Cannabis-based medicinal products
- NHS: Medical cannabis (and cannabis oils)
- MHRA: The supply, manufacture, importation and distribution of unlicensed cannabis-based products for medicinal use