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.

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. It is also why the two words are not interchangeable, even though they are often lumped together as "cannabis" in everyday conversation. When someone asks whether cannabis is safe, or whether it works, the honest answer usually depends on which compound is meant, in what amount, and for whom.

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.

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. CBD carries its own considerations, mainly its interactions with other medicines and its effects on the liver, but these are managed rather than reasons a specialist would usually decline treatment outright.

It is also worth being clear about the limits of the evidence for both. Outside the small number of licensed uses, much of what is claimed for either compound rests on early-stage or lower-quality studies, and the picture varies a great deal from one condition to the next. That uncertainty is not a reason to dismiss the field, but it is a reason to be sceptical of anyone who speaks about either compound in absolute terms. What the evidence supports condition by condition is set out in Can medical cannabis treat my condition?

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?

How the difference shapes treatment in practice

Because the two compounds carry such different risks, the balance between them is one of the first things a specialist considers, and it is a large part of why treatment is introduced slowly rather than at a full dose from the start.

Treatment usually begins only after a specialist has reviewed your diagnosis, the treatments you have already tried and your wider medical history. Where a cannabis-based product is judged appropriate, it is typically started at a low dose and increased gradually — a process known as titration — so that any benefit can be weighed against side effects before the dose goes up again. Much of the caution in that process is really about managing exposure to THC, since that is the component most likely to cause impairment, anxiety or a raised heart rate; the CBD component is handled with care too, chiefly because of its interactions with other medicines.

The medicines themselves usually come as oils or capsules taken by mouth, or as dried flower for vaporising, and the format changes how quickly effects are felt and how long they last. How the balance of compounds, the format and the dose are chosen and then adjusted is set out in How is a dose decided and adjusted? and How are cannabis-based medicines taken?

Follow-up reviews then check whether the treatment is helping and whether side effects are tolerable, and the plan is changed accordingly. If a product is not helping, or the side effects outweigh the benefit, the honest course is to adjust it or stop — being prescribed a medicine is not a commitment to stay on it.

What the difference means day to day

The practical consequences of the THC and CBD distinction reach well beyond the consultation, most obviously where driving, work and travel are concerned.

THC can impair driving and is subject to a specified blood limit under road traffic law, whereas CBD is not intoxicating and is not subject to the same limit — though the separate offence of driving while impaired applies to any medicine. What this means for someone holding a prescription is covered in Driving with a medical cannabis prescription, and the related questions of workplace testing are discussed in Medical cannabis, drug testing and work.

It is also worth telling your GP about any cannabis-based treatment so that it sits on your records alongside your other medicines, particularly given CBD's interactions. That conversation is set out in Telling your GP about a medical cannabis prescription.

Patient guide · Written by the Flora House editorial team · Last reviewed: 7 October 2026