Understanding Medical Cannabis: Mechanisms, Indications and Clinical Integration

1. Definition

Medical cannabis refers to cannabis-derived pharmacological products prescribed by qualified healthcare professionals for the treatment of specific medical conditions. These preparations differ significantly from illicit or recreational cannabis in that they are produced under pharmaceutical standards, contain defined concentrations of active compounds, and are prescribed within legal and regulatory frameworks through specialist clinics.

2. Indications for Use

Current evidence and prescribing practice support the use of medical cannabis for the following conditions, particularly in patients who have not responded to conventional therapies:

• Chronic, non-cancer pain, including neuropathic and musculoskeletal pain¹
• Refractory epilepsy, particularly in paediatric cases² (e.g., Dravet or Lennox-Gastaut syndromes). It is rare for adult patients to be prescribed medical cannabis primarily as anti-epilepsy treatment, primarily due to cost implications.
• Mood disorders, anxiety-related disorders and post-traumatic stress disorder (PTSD)³
• Chemotherapy-induced nausea and vomiting (CINV)⁴
• Spasticity and symptom management in multiple sclerosis and other neurological conditions.⁵

These patient populations typically exhibit high symptom burden and limited therapeutic response to existing NHS-approved pharmacological options.

3. Mechanism of Action

The therapeutic effects of medical cannabis are primarily mediated via the endocannabinoid system (ECS)—a neuromodulatory system involved in maintaining homeostasis across the central and peripheral nervous systems, immune system, and gastrointestinal tract.

Receptor Targets

• CB1 receptors: Predominantly expressed in the central nervous system; modulate pain, appetite, memory, and mood.
• CB2 receptors: Primarily located in immune cells and peripheral tissues; involved in inflammation and immune modulation.

Principal Active Compounds

• Δ9-Tetrahydrocannabinol (THC): A partial agonist at CB1 and CB2 receptors; possesses analgesic, antiemetic, and orexigenic (appetite stimulant) effects but also confers psychoactive risks.
• Cannabidiol (CBD): Exhibits low affinity for CB1/CB2 but acts as a negative allosteric modulator of CB1, dampening the psychoactive potential of THC; also interacts with non-cannabinoid targets (e.g., 5-HT1A, TRPV1 receptors).

When administered in balanced THC:CBD ratios, medical cannabis may offer symptom relief with an improved safety profile compared to monotherapy or street cannabis.

In addition, other biologically active compounds (terpenes and flavonoids amongst others) also exert effect beyond that offered by THC and CBD. You can have two 20% THC strains of cannabis yet one will lead to a focussed, creative patient while the other leads to a very sedate, relaxed patient ready for bed. It stands to reason that there is more to cannabis than just THC and CBD.

4. Legal and Regulatory Status (UK)

Since November 2018, medical cannabis has been reclassified in the UK as a Schedule 2 controlled substance under the Misuse of Drugs Regulations 2001, permitting prescribing by specialist physicians.

• Access pathways: Primarily through CQC regulated private sector clinics, as NHS prescribing remains extremely limited.
• NHS-licensed products: Include Epidyolex (CBD oral solution) for severe epilepsy; Sativex for MS-related spasticity. Dronabinol and Marinol are also cannabinoid medications listed in the BNF.
• Off-license prescribing: Common in private clinics, subject to clinical justification and documentation.

Despite legal reforms, awareness and education among NHS clinicians remain low, often leading to the mislabelling of patients’ intention of use of their prescription.

5. Barriers to Integration in Mainstream Practice⁶

Patient-Reported Issues

Patients with valid prescriptions frequently report:

• Misclassification of medical cannabis as illicit substance
• Confiscation or denial of access during hospital admissions
• Negative stereotyping or accusations of substance misuse
• Breakdown in communication between private prescribers and NHS teams.

These experiences contribute to patient distress, loss of trust, and avoidance of NHS healthcare settings.

Clinician-Related Barriers

Common causes for clinical hesitancy include:

• Lack of high-quality RCTs due to the inability to patent a plant
• Fragmentation between public (NHS) and private sector governance
• Limited understanding of endocannabinoid system (ECS) pharmacology among general clinicians

Countries such as Germany, Canada, and Poland have implemented more integrated models of medical cannabis prescribing, whereas UK practice remains restrictive and siloed. Introducing understanding of the ECS at medical school level would go some way to dispelling the narrative that cannabis just causes harm.

6. Implications for Clinical Practice

Medical cannabis is legal for select indications, particularly in refractory cases. Clinicians should be aware of the following:

• Cannabis-based medicinal products (CBMPs) are regulated, titratable therapies, not interchangeable with recreational cannabis.
• Patients pursuing these treatments have often exhausted first- and second-line options within the NHS.
• Improved clinician awareness of prescribing frameworks, drug interactions, and ECS physiology can significantly enhance continuity of care and therapeutic alliance.

There are significant differences between medical and ‘street’ cannabis in terms of quality/purity. By medically being in control of a prescription not only does it become easy to monitor usage to counter any concerns about misuse or adverse psychological side effects, it also becomes possible to prescribe an appropriate strain or formulation for a particular scenario (e.g. ensuring a focussing strain is prescribed for patients with ADHD; a strain that has strong anti-inflammatory properties for an axial spondyloarthropathy). This clearly is preferable to the blind roulette that patients would otherwise have to negotiate with street cannabis, let alone the legal implications.

Concerns about cannabis induced psychosis are legitimate, however in reality, of those patients who are prescribed it via the plethora of private clinics here in the UK, the number of psychotic events is not elevated compared to the background rate of psychosis within the general population.⁷

With greater numbers of patients being prescribed medical cannabis and with a proportion of those likely to present to Emergency Departments, it is vital that we as emergency clinicians educate ourselves about this new emerging treatment option.

References

1. Solmi M, et al. Balancing risks and benefits of cannabis use: umbrella review of meta-analyses of randomised controlled trials and observational studies. BMJ 2023;382:e072348.
2. Ben-Zeev B. Medical Cannabis for Intractable Epilepsy in Childhood: A Review. Rambam Maimonides Med J. 2020 Jan 30;11(1):e0004. 
3. Pillai M, Erridge S, Bapir L, et al. Assessment of clinical outcomes in patients with post-traumatic stress disorder: analysis from the UK Medical Cannabis Registry. Expert Rev Neurother. 2022 Nov-Dec;22(11-12):1009-1018.
4. Bathula PP, Maciver MB. Cannabinoids in Treating Chemotherapy-Induced Nausea and Vomiting, Cancer-Associated Pain, and Tumor Growth. Int J Mol Sci. 2023 Dec 20;25(1):74.
5. Haddad F, et al. The Efficacy of Cannabis on Multiple Sclerosis-Related Symptoms. Life (Basel). 2022 May 5;12(5):682.
6. Known Unknowns of Medical Cannabis, 2022.
7. Psychosis data report. Public Health England. Gov.uk 2016.