With medical cannabis now legal in 44 countries around the world, and the UK likely to follow suit, here is what pharmacists need to know.

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Cannabis plants contain more than 100 cannabinoids, as well as other compounds like flavonoids and terpenoids, which is thought to be why some people respond better to herbal cannabis than pure cannabis derivatives. Cannabinoids act on the body’s endocannabinoid system, which helps regulate many bodily functions via CB1 receptors, found mainly in the brain; CB2 receptors in immune cells; the gastrointestinal tract; and peripheral nervous system. The two most studied cannabinoids are THC and CBD.

THC (δ-9 tetrahydrocannabinol) THC is the psychoactive component of cannabis. Weak partial agonist on CB1 and CB2 receptors. THC potency in dried cannabis has increased from an average of 3% in the 1980s to around 15% today. Some strains contain more than 25% potency

CBD (cannabidiol) CBD does not produce psychoactive adverse effects. Has little affinity for CB1 and CB2 receptors directly. It is believed to modulate the effects of THC throughout the endocannabinoid system which is why the THC/CBD ratio can lead to different responses and adverse effects

Show Fullscreen Figure 1: Evidence for medical cannabis and potential adverse effects There is conclusive or substantial evidence for the use of medical cannabis in treating intractable seizures in Dravet and Lennox-Gastaut syndrome, chemotherapy-induced nausea and vomiting, improving symptoms in multiple sclerosis and treating chronic pain in adults, however the evidence regarding other indications is moderate or limited and there are several potential side effects.

Show Fullscreen Figure 2: Routes of cannabis administration and dosing *using a vaporising device that blows hot air through finely ground cannabis at a specified temperature There are no established uniform dosing schedules for products such as fresh marijuana, smoked/vaporised marijuana or cannabis oil. Patients should start with a very low dose, e.g. 1mg THC, and titrate slowly until the desired effect is achieved, stopping if unacceptable adverse effects occur.