Aim:

To review the current evidence regarding the effectiveness of cannabinoids in the treatment of chronic pain.

Data sources:

MEDLINE/PubMed, Cochrane Library, TRIP Database, National Guideline Clearing House, Canadian Medical Association Practice Guidelines.

Methods:

In August 2019, we searched for meta-analysis (MA), systematic reviews (SR), observational studies (OS), clinical trials (CT), and guidelines, published in Portuguese and English, with no time limit, using the MeSH terms ‘cannabis’ and ‘chronic pain’. The studies that were included were the ones regarding cannabinoid treatment for chronic pain of any cause in adults. Any study regarding acute pain was excluded. The Strength of Recommendation Taxonomy scale, from the American Academy of Family Physicians, was applied to assign levels of evidence (LE) and strength of recommendation (SOR).

Results:

A total of 244 articles were obtained, and 16 of those fulfilled the inclusion criteria: nine SR, four randomized double-blind CT, two retrospective case-series studies, and one prospective case-series study. All studies focused on chronic pain, but the etiology varied between studies (oncologic pain, neuropathic pain, rheumatologic pain, visceral pain). The results were not consistent between the studies. Although there may be some benefits to neuropathic pain, the reviews are all in agreement that greater dimension and longer duration CT are necessary for cannabinoids usage to have robust evidence. Gastrointestinal as well as motor and cognitive function adverse effects may occur, especially with preparations containing a higher dosage of tetrahydrocannabinol. There is no evidence for treatments with cannabinoids in rheumatologic pain. LE 1 was not assigned to any of the included studies.

Conclusions:

The therapeutic usage of cannabinoids, while promising and with likely benefit identified in some small studies for some types of chronic pain (especially neuropathic), has limited evidence (SOR B), and requires some higher quality, bigger studies. The possible long-term efficacy and side effects should be considered on longer clinical trials, which may be achieved with the rise of the prescriptions of these compounds. Regarding the current evidence, cannabinoids can be the last resort solution in cases of refractory neuropathic and oncologic pain.

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