THC vs CBD — What’s the Difference for Medical Patients?
A complete guide to the two primary cannabinoids in medical cannabis — how they work, what conditions they treat, and how to choose the right one for your prescription.
THC vs CBD at a Glance
The two most clinically significant cannabinoids in medical cannabis behave very differently in the body. Here is a direct comparison of the properties that matter most to UK medical patients.
| Property | THC | CBD |
|---|---|---|
| Full name | Tetrahydrocannabinol | Cannabidiol |
| Psychoactive? | Yes — produces euphoria and altered perception | No — non-intoxicating at any dose |
| Primary receptor | CB1 (brain & CNS) agonist | CB1 negative modulator; CB2 partial agonist |
| Medical uses | Pain, spasticity, nausea, appetite stimulation, PTSD, sleep | Epilepsy, anxiety, inflammation, sleep, neuroprotection |
| Side effects | Anxiety, paranoia, impaired memory, dry mouth, tachycardia | Generally well tolerated; possible fatigue, diarrhoea at high doses |
| UK legal status | Schedule 2 — prescription only | Schedule 2 — prescription only (as CBPM) |
| Typical % in flower | 15–30% | 5–20% |
| Tolerance development | Moderate — CB1 receptor downregulation with chronic use | Low — more stable long-term dosing |
| Drug driving risk | Yes — impairs driving; zero-tolerance UK limit | Low — but products may contain trace THC |
Both THC and CBD are available as cannabis-based products for medicinal use (CBPMs) in the UK. They require a prescription from a GMC-registered specialist doctor.
THC (Tetrahydrocannabinol)
THC is the primary psychoactive constituent of cannabis and one of the most pharmacologically active plant-derived compounds known. It is the main reason cannabis has been used medicinally for thousands of years — and the reason it is strictly regulated.
How THC Works
THC is a partial agonist at CB1 receptors — the primary endocannabinoid receptors concentrated in the brain, spinal cord and peripheral nervous system. When THC binds to CB1 receptors, it mimics the body’s natural endocannabinoid anandamide (known informally as the “bliss molecule”), producing a cascade of effects including pain relief, euphoria, relaxation, and altered sensory perception.
CB1 receptors are especially dense in the basal ganglia (movement), cerebellum (coordination), hippocampus (memory), prefrontal cortex (cognition) and limbic system (emotion) — which explains both THC’s therapeutic effects on pain, nausea and muscle spasticity, and its side-effect profile including memory impairment, anxiety and psychoactivity.
Medical Conditions Treated with THC
- Chronic pain — including neuropathic, cancer-related and musculoskeletal pain; THC reduces central pain sensitisation via CB1 modulation
- Multiple sclerosis spasticity — Sativex (nabiximols), the first licensed cannabis medicine in the UK, is a 1:1 THC:CBD spray used specifically for MS spasticity
- Chemotherapy-induced nausea and vomiting (CINV) — THC’s antiemetic action works via CB1 receptors in the brainstem
- PTSD — emerging evidence suggests THC may reduce nightmare frequency and improve sleep in PTSD patients by modulating fear memory consolidation
- Appetite and weight loss — THC’s orexigenic (appetite-stimulating) effect is well established; used in cancer cachexia and AIDS wasting syndrome
- Insomnia — THC reduces sleep latency and may increase slow-wave sleep, though long-term effects on sleep architecture are complex
THC in UK Medical Cannabis Prescriptions
In the UK, THC-containing cannabis products are Schedule 2 controlled drugs, meaning they can only be prescribed by a specialist doctor (not a GP). The prescription must be written on a Controlled Drug prescription form. Patients must store their medication securely and cannot travel internationally with it without special authorisation.
THC flower products available in the UK typically contain 15–30% THC, expressed as a percentage of dry weight. Oils usually contain THC in mg/ml alongside CBD, at ratios tailored to specific indications.
High-THC Products Available in the UK (20%+)
CBD (Cannabidiol)
CBD is the second most abundant cannabinoid in most medical cannabis varieties and the subject of intense pharmaceutical research. Unlike THC, it is non-intoxicating and is often preferred by patients who require symptom relief without cognitive impairment.
How CBD Works
CBD’s pharmacology is considerably more complex than THC’s. Rather than acting as a direct agonist at cannabinoid receptors, CBD works through multiple mechanisms simultaneously. It acts as a negative allosteric modulator at CB1 receptors — meaning it can reduce the potency of THC when both are present. It is also a partial agonist at CB2 receptors (concentrated in immune tissue), explaining its anti-inflammatory properties.
Beyond cannabinoid receptors, CBD is a potent agonist at serotonin 5-HT1A receptors (anxiolytic and antidepressant effects), an antagonist at GPR55 receptors (implicated in bone density and cancer cell proliferation), and an inhibitor of anandamide reuptake — increasing the body’s endogenous cannabinoid tone. Its anticonvulsant effect is thought to involve sodium channel modulation and TRPV1 activation.
Medical Conditions Treated with CBD
- Treatment-resistant epilepsy — Epidyolex, a pharmaceutical-grade CBD oral solution, is NICE-approved for Dravet syndrome and Lennox-Gastaut syndrome in the UK
- Anxiety disorders — multiple clinical trials demonstrate CBD’s anxiolytic effects at doses of 300–600mg; well-suited for patients who cannot tolerate THC-induced anxiety
- Inflammatory conditions — CBD’s CB2 activity and cytokine modulation offer anti-inflammatory benefit relevant to conditions like arthritis and IBD
- Sleep disorders — at lower doses CBD may have wake-promoting effects; at higher doses it appears to reduce anxiety that disrupts sleep, improving sleep quality indirectly
- Neuropathic pain — often used in combination with THC; CBD may reduce the dose of THC needed for effective pain relief, thereby reducing side effects
- PTSD — early evidence suggests CBD may block the reconsolidation of fear memories, complementing any THC prescribed for the same condition
Why Some Patients Prefer CBD-Dominant Products
For patients in safety-critical occupations, those who drive regularly, or those with a personal or family history of psychosis, CBD-dominant prescriptions offer therapeutic benefit with minimal impairment risk. CBD also lacks the tolerance development pattern seen with THC, allowing more stable long-term dosing. Many patients start on CBD-dominant products and have THC added incrementally as their specialist assesses their response.
CBD-Dominant Products Available in the UK
The Entourage Effect: How THC and CBD Work Together
The entourage effect, first proposed by Israeli pharmacologist Raphael Mechoulam in 1998, describes the phenomenon whereby the full spectrum of cannabis compounds — cannabinoids, terpenes and flavonoids — produce greater therapeutic benefit together than any single compound alone. The THC–CBD relationship is the most clinically studied example.
CBD is now well established as a modulator of THC’s effects: it reduces THC-induced anxiety and paranoia, attenuates memory impairment, and may reduce the intoxication ceiling. This is why many UK medical cannabis specialists prescribe products containing both rather than pure THC isolates. The ratio of THC to CBD fundamentally shapes the clinical profile of the product.
Terpenes add a further dimension — myrcene enhances sedation, limonene lifts mood, pinene may counteract THC-related memory impairment, and linalool contributes anxiolytic effects. A product’s full chemical profile, not just its THC and CBD percentages, determines its therapeutic effect.
Strong analgesic and sedative profile; more psychoactivity. Typically used for severe pain, MS spasticity, PTSD and appetite stimulation.
CBD moderates THC’s psychoactivity. The ratio used in Sativex. Suited to pain, inflammation and patients new to THC.
Minimal intoxication. Anti-inflammatory, anxiolytic and anticonvulsant profile. Used for epilepsy, anxiety and daytime treatment.
Which Should I Choose? A Guide by Condition
This is a clinical decision — always work with your specialist. The following reflects general prescribing practice and published evidence for common indications. Individual patient factors (tolerance, comorbidities, occupation, other medications) will affect recommendations.
Chronic Pain
THC or BalancedHigh THC or a balanced 1:1 ratio is most commonly prescribed for chronic pain. THC’s CB1 action directly reduces central pain sensitisation. CBD adds anti-inflammatory benefit. Many patients use high-THC flower at night and a lower-THC oil during the day.
Anxiety Disorders
CBD-DominantCBD-dominant products (10:1 CBD:THC or higher) are first-line for anxiety. High-THC products frequently worsen anxiety, especially in new patients. Low-dose THC may be added once CBD tolerance is established and only under specialist supervision.
Epilepsy
CBD-DominantCBD is the only cannabinoid with a NICE-approved licensed medicine for epilepsy in the UK (Epidyolex). CBD-dominant prescriptions are standard for treatment-resistant epilepsy, particularly Dravet and Lennox-Gastaut syndromes.
Sleep Disorders
THC-DominantTHC-dominant indica-type flower is most commonly used for sleep. THC reduces sleep latency and increases slow-wave sleep. CBD may be alerting at lower doses. A moderate THC dose taken 60–90 minutes before bed is typical.
MS Spasticity
Balanced or High THCSativex (1:1 THC:CBD) is the licensed medicine for MS spasticity in the UK. Unlicensed balanced CBPMs are also prescribed. The combined action on CB1 (spasm suppression) and anti-inflammatory CB2 pathways makes the balanced ratio particularly effective.
Nausea (CINV)
THC-DominantTHC is the primary antiemetic cannabinoid, acting via CB1 receptors in the brainstem’s vomiting centre. UK prescriptions for chemotherapy-induced nausea are typically THC-dominant with or without CBD to moderate side effects.
Use the LeafMe Dosage Calculator to explore starting doses by condition ›
Frequently Asked Questions
Is THC or CBD better for chronic pain?
Both cannabinoids contribute to pain relief through different mechanisms. THC activates CB1 receptors and reduces pain perception centrally, while CBD modulates inflammation and peripheral pain signalling. Many UK medical cannabis patients with chronic pain are prescribed products containing both — a 1:1 THC:CBD ratio is commonly used as it may provide synergistic benefit while CBD partially moderates THC's psychoactive effects. Your specialist will assess your pain type, history and tolerance to recommend the most appropriate ratio.
Can you get CBD on prescription in the UK?
Yes. CBD-dominant and balanced cannabis products are legally available on specialist medical cannabis prescriptions in the UK. Since November 2018, cannabis-based products for medicinal use (CBPMs) can be prescribed by specialist doctors registered with the GMC. This includes flower, oils and other preparations with significant CBD content. Over-the-counter CBD products sold as food supplements are a different category and are not equivalent to prescribed medical cannabis.
What CBD:THC ratio is best for anxiety?
For anxiety, most specialists recommend CBD-dominant or high-CBD products with low or trace levels of THC. High-THC products can worsen anxiety and trigger paranoia in susceptible patients, particularly at higher doses. A ratio of 10:1 CBD:THC or higher is often the starting point for anxiety conditions. Balanced 1:1 products may suit patients who also have comorbid pain or sleep issues. Your prescribing clinician will tailor the ratio to your specific anxiety type and overall clinical picture.
Does CBD get you high?
No. CBD is non-intoxicating. It does not bind strongly to CB1 receptors in the brain — the mechanism through which THC produces psychoactive effects. CBD on its own does not impair cognition, produce euphoria or cause the "high" associated with recreational cannabis use. Some patients do notice a calming or relaxing sensation, but this is distinct from intoxication. Products that are genuinely CBD-dominant and contain only trace THC (under 0.2%) are considered non-intoxicating.
How do I know whether I need THC or CBD?
This is a clinical decision best made with a specialist doctor. As a general guide: THC-dominant products tend to suit patients with chronic pain, spasticity, nausea, PTSD and severe sleep disorders. CBD-dominant products are often preferred for epilepsy, anxiety, inflammation and conditions where psychoactivity is unwanted. Balanced products suit many patients who need both analgesic and anxiolytic benefit. Your medical history, other medications and individual response will all influence the recommendation.
Does tolerance develop to THC and CBD differently?
Yes. Tolerance to THC develops relatively quickly with regular use, primarily through CB1 receptor downregulation. This is why THC doses may need adjustment over time. CBD tolerance is less well characterised but appears to be lower — some research suggests CBD may actually reset or modulate THC tolerance. Clinical practice in the UK generally recommends starting at the lowest effective dose of THC and titrating slowly to minimise tolerance development, while CBD doses are typically more stable over time.
Explore Further
Deepen your understanding of UK medical cannabis with our specialist guides.