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Can marijuana treat chronic pain? Studies say evidence insufficient

Stephen Padilla
17 Aug 2017
Australian lawmakers are looking into legalising marijuana for medical purposes, following the cue of Mexico.

There is limited evidence showing the efficacy of cannabis in alleviating neuropathic pain, but evidence for other types of chronic pain is lacking, according to a systematic review. Evidence is also insufficient with respect to its association with an increased risk for nonserious short-term adverse effects and potentially serious mental health adverse effects, such as psychosis.

Researchers accessed Medline, Cochrane Database of Systematic Reviews and other sources from inception to March 2017 to review the benefits of plant-based cannabis preparations for treating chronic pain in adults and the harms of cannabis use in chronic pain and general adult populations. Studies selected were intervention trials and observational studies published in English, involving adults using plant-based cannabis preparations that reported pain quality of life or adverse effects.

Low-strength evidence from 27 chronic pain trials suggested that cannabis relieves neuropathic pain. However, evidence for other pain populations was inadequate. [Ann Intern Med 2017;doi:10.7326/M17-0155]

Eleven systematic reviews and 32 primary studies revealed that cannabis use in general populations may cause harms such as increased risk for motor vehicle accidents, psychotic symptoms and short-term cognitive impairment. There were no adverse pulmonary effects seen in younger populations, but evidence on most other long-term physical harms, in heavy or long-term cannabis users, or in older populations was lacking.

“[W]e found limited evidence on the potential benefits and harms of cannabis use in chronic pain populations,” researchers said. “Most studies are small, many have methodological flaws, and the long-term effects are unclear given the brief follow-up of most studies.”

These findings support several recent studies. One review concluded that there was low- to moderate-strength evidence suggesting the efficacy of cannabis in chronic pain populations, particularly among those with multiple sclerosis or neuropathic pain. This was refuted, however, by a separate group that reviewed and reanalysed a similar set of published articles. The authors found that insufficient to low-strength evidence supports cannabis use in treatment of chronic noncancer pain. [www.health.state.mn.us/topics/cannabis/intractable/medicalcannabisreport.pdf]

Interestingly, a recent report that investigated the biological and clinical effects of cannabis across a broad range of indications found substantial evidence of benefit for chronic pain. [National Academies of Sciences, Engineering, and Medicine. The Health Effects of Cannabis and Cannabinoids: The Current State of Evidence and Recommendations for Research. Washington, DCNational Academies Pr. 2017]

Although the overall conclusions seem to differ from our findings, the authors stipulated that the clinical improvements were modest and limited to neuropathic pain, and they underscored the urgent need for better research clarifying the effects of cannabis,” researchers said.

The current review supplemented this report by the use of a systematic approach on a more focused topic, namely chronic pain and harms, as well as standard terminology for describing the strength of the body of evidence. [www.effectivehealthcare.ahrq.gov/ehc/products/457/1752/methods-guidance-grading-evidence-131118.pdf]

“Even though we did not find strong, consistent evidence of benefit, clinicians will still need to engage in evidence-based discussions with patients managing chronic pain who are using or requesting to use cannabis,” researchers noted. “Therefore, clinicians must understand what is known and unknown about its potential harms.”

This review has several limitations: (1) there are few methodologically rigorous trials; (2) the cannabis formulations studied may not reflect commercially available products; and (3) applicability to older, chronically ill populations and patients who use cannabis heavily is limited.

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