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Medical Cannabis in the Treatment of Chronic Pain – A Review of the Evidence

Medical Cannabis in the Treatment of Chronic Pain – A Review of the Evidence

Medical cannabis, known more commonly in some jurisdictions as medical marijuana, is becoming more widely available in many countries around the world for a wide variety of conditions. Countries including the US, Canada, the UK, Germany, and Australia have all legalised the prescription of cannabis-based medicines for conditions such as epilepsy, Parkinson’s Disease, and Multiple Sclerosis. However, the most common reason for medical cannabis prescriptions is chronic pain.

Chronic pain refers to a number of conditions which, according to the British Pain Society, are estimated to affect around 43% of the UK population – of which 14.3% of cases would be described as either moderately or severely debilitating. Despite this high prevalence rate, current pharmaceutical treatments are limited, and in some cases ineffective. Patients and clinicians alike are increasingly considering the potential of medical cannabis as an alternative treatment option. However, there remains a relative lack of clinical evidence on the subject.

A recent article aimed to review the existing scientific evidence related to medical cannabis for the treatment of chronic pain in order to update physicians on relevant and optimal prescribing practices.

About the Study

The review was recently published in the Australian Journal of General Practice with a focus on the future treatment options for medical cannabis in Australian chronic pain patients. It is estimated that around 600,000 Australians currently self-medicate with cannabis, with chronic pain being a leading reason for its use. The majority of self-medication involved illicit cannabis – a pattern also mirrored in the UK.

Medical cannabis has been available for prescription in Australia since late 2016 with more than 190 different products now accessible via the Therapeutic Goods Administration (TGA) schemes. Over 130,000 prescriptions for medical cannabis were filled in Australia up until July 2021, with approximately 65% of these for the treatment of chronic pain.

Methods and Design of the Study

According to the authors, this review was designed to “examine the use of tetrahydrocannabinol (THC) and cannabidiol (CBD) in the treatment of chronic pain and summarise the evidence related to efficacy, side effects and optimal prescribing practices.” While the researchers note that there have been several other reviews in this area, many of these use evidence of low quality. Furthermore, a recent ‘review of reviews’ concluded that the 57 systematic reviews of the literature over the past 20 years were “lacking in quality and cannot provide a bases for (clinical) decision making”.

The researchers categorised the existing evidence into forms and types of medical cannabis and side effects to determine the potential of cannabis-based products for the treatment of a variety of chronic pain conditions.

Results of the Review

Inhaled cannabis (usually herbal cannabis/cannabis flower) is one of the most popular forms of medical cannabis, accounting for almost 30% of SAS-B approvals in Australia. Medical cannabis in this form is usually vaporised rather than smoked to reduce the potential harms.

Current evidence shows that inhaled cannabis may be associated with improvements in conditions such as diabetic and human immunodeficiency virus–related neuropathies and cases of neuropathic pain. Furthermore, a large observational study of thousands of Israeli patients presented improvements in cancer-related pain and quality of life across a period of several months.

Researchers also identified positive outcomes in the use of Nabiximols (THC/CBD oromucosal spray). Past studies showed that the product was associated with positive effects in patients with brachial plexus avulsion and mixed peripheral neuropathic pain. The findings of a German patient registry (800 patients) also indicated that 70% of patients reported a >50% improvement in pain after 12 weeks. However, the results of other studies have been inconclusive and, in some cases, negative (pain associated with spinal cord injury, diabetes, and chemotherapy).

The researchers note that there have been relatively few studies assessing the analgesic effects of CBD in humans. The clinical evidence that does exist is mixed with some studies showing that “CBD had no clear analgesic effect in laboratory tests of pain thresholds and sensitivity”. On the other hand, other studies have shown that CBD may be effective in improving symptoms of chronic neuropathic pain, non-cancer pain; however, these improvements were more often associated with quality-of-life measures, including anxiety, stress, and depression.

Conclusions

Having reviewed the existing evidence, the authors of this review concluded that “medicinal cannabis is worthy of consideration in the management of chronic pain, and it is important that doctors are aware of the positives and negatives related to its use.” However, the authors concede that much of the existing evidence is of low quality and recommend that more high-quality evidence is required in this area.

Nonetheless, millions of patients around the world – including tens of thousands in Australia – are receiving medical cannabis prescriptions for chronic pain conditions, with millions more self-medicating with illicit products. The researchers therefore recommend that more clinical trials are carried out to improve the understanding of clinicians and patients in the potential of medical cannabis products for the treatment of a wide variety of chronic pain conditions.