Evidence behind the effectiveness of cannabis-related products to treat chronic pain is ‘surprisingly thin’, according to a new systematic evidence review by researchers at Oregon Health & Science University (OHSU).

The federally funded review, which will be updated on an ongoing basis, was published in the Annals of Internal Medicine.

Researchers did find evidence to support short-term benefit in treating neuropathic pain – caused by damage to peripheral nerves, such as diabetic neuropathy resulting in pain described as burning and tingling – with two FDA-approved synthetic products containing 100% tetrahydrocannabinol (THC): dronabinol and nabilone. Side effects included sedation and dizziness, according to the review.

Another product also showed evidence of some clinical benefit for neuropathic pain: a spray applied under the tongue comprising equal parts THC and cannabidiol, or CBD, extracted from the cannabis plant, known as nabiximols. The product also led to side effects, such as nausea, sedation and dizziness.

‘In general, the limited amount of evidence surprised all of us,’ said lead author Marian S. McDonagh, emeritus professor of medical informatics and clinical epidemiology in the OHSU School of Medicine, saying they found evidence for 'short-term reduction in chronic pain, but little else'.

‘With so much buzz around cannabis-related products, and the easy availability of recreational and medical marijuana in many states, consumers and patients might assume there would be more evidence about the benefits and side effects.

‘Unfortunately, there is very little scientifically valid research into most of these products,’ the researchers claimed.

For some cannabis products, such as whole-plant products, the data was ‘sparse with imprecise estimates of effect’ and studies had ‘methodological limitations’, the authors suggested.

Co-author Roger Chou, director of OHSU’s Pacific Northwest Evidence-based Practice Center and professor of medical informatics and clinical epidemiology, and medicine (general internal medicine and geriatrics) in the OHSU School of Medicine, said products varied ‘quite a bit’ in terms of their chemical composition, which ‘could have important effects in terms of benefits and harm to patients’.

The Pharmacist has previously spoken to pharmacists in the UK about the importance of the use of high quality cannabis products in pain management services.