Medical cannabis: what the evidence says

What medical cannabis is

The cannabis plant contains many active chemicals called cannabinoids. Two are talked about most: THC, which produces the intoxicating high and has some medicinal effects, and CBD, which does not cause a high. Medical cannabis is an umbrella term for cannabis-based products used for a medical reason, and these vary a great deal. At one end are licensed medicines that have been through formal testing and approval for specific conditions, containing defined amounts of cannabinoids. At the other are unlicensed cannabis-based products, which have not gone through the same full approval process. This is different again from CBD wellness products sold in shops, which are food supplements and are not medicines. Because the term covers such different things, it is important to be clear about exactly which type of product is being discussed, as their evidence, quality control and legal status are not the same.

The UK legal and prescribing position

In November 2018, the law in the UK changed so that specialist doctors on the General Medical Council's specialist register could legally prescribe cannabis-based products for medicinal use in certain situations. Importantly, this did not make cannabis freely available: it can only be prescribed by a specialist, not a GP, and only when the patient has an unmet clinical need that established treatments have not helped. National guidance recommends these products for only a small number of situations where there is reasonable evidence, and NHS prescriptions remain uncommon. Some people obtain unlicensed products through private clinics. Recreational cannabis remains illegal. Low-strength CBD supplements sold in shops are legal as food products but are not permitted to make medical claims. The overall picture is one of tightly controlled, specialist-led prescribing rather than broad availability.

Where the evidence supports use

The strongest evidence for cannabis-based medicines is in a small number of specific situations. Certain licensed products are recommended for rare, severe forms of childhood epilepsy that have not responded to other treatments, where trials have shown a reduction in seizures for some children. A mouth spray combining THC and CBD is used for muscle stiffness and spasms in multiple sclerosis when other treatments have not helped enough. A cannabis-based medicine is also used to help with sickness caused by chemotherapy when standard anti-sickness treatments have not worked. In these areas, the benefit for selected patients is supported by better-quality research, which is why they feature in national guidance. Even here, the products are used carefully, under specialist supervision, and are not suitable for everyone, but they represent the situations where evidence is currently strongest.

Where the evidence is weak or uncertain

For many of the conditions people hope cannabis will help, the evidence is limited, mixed or of poor quality. Chronic pain is the most common reason people seek medical cannabis, yet current UK guidance does not recommend it for most chronic pain because the evidence of meaningful benefit is weak and uncertain, and any benefit must be weighed against side effects. Evidence is also limited for anxiety, depression, sleep problems and many other conditions for which cannabis is often promoted. Much of the research is small, short-term or of low quality, making firm conclusions difficult, and marketing claims often run ahead of the science. This does not mean these products never help anyone, but it does mean people should be cautious about strong claims, and that more good-quality research is needed before cannabis can be recommended widely for these uses.

Risks and things to consider

Cannabis-based products are not harmless, and the risks need weighing against any hoped-for benefit. Short-term effects can include dizziness, tiredness, dry mouth, changes in appetite, low mood or anxiety, and, with THC-containing products, feeling high or impaired, which can affect driving and daily tasks. There are concerns about longer-term effects on mental health, particularly with regular use of higher-THC products, and about dependence. Cannabis can interact with other medicines, and it may not be suitable in pregnancy, in young people whose brains are still developing, or for those with a history of certain mental health conditions. Unlicensed and unregulated products can vary in strength and quality. Anyone considering cannabis for a medical reason should discuss it with a doctor, be wary of exaggerated online claims, and never stop or replace prescribed treatment without advice. Buying cannabis illegally carries additional legal and safety risks.