MD, PhD, FMedSci, FRSB, FRCP, FRCPEd.

Cannabis seems often to be an emotional subject where more heat than light is generated. Does it work for chronic pain? This cannot be such a difficult question to answer definitively. Yet, systematic reviews have provided conflicting results due, in part, to limitations of analytical approaches and interpretation of findings.

A new systematic review is therefore both necessary and welcome. It aimed at determining the benefits and harms of medical cannabis and cannabinoids for chronic pain. Included were all randomised clinical trials of medical cannabis or cannabinoids versus any non-cannabis control for chronic pain at ≥1-month follow-up.

A total of 32 trials with 5174 adult patients were included, 29 of which compared medical cannabis or cannabinoids with placebo. Medical cannabis was administered orally (n=30) or topically (n=2). Clinical populations included chronic non-cancer pain (n=28) and cancer-related pain (n=4). Length of follow-up ranged from 1 to 5.5 months.

Compared with placebo, non-inhaled medical cannabis probably results in a small increase in the proportion of patients experiencing at least the minimally important difference (MID) of 1 cm (on a 10 cm visual analogue scale (VAS)) in pain relief (modelled risk difference (RD) of 10% (95% confidence interval 5% to 15%), based on a weighted mean difference (WMD) of −0.50 cm (95% CI −0.75 to −0.25 cm, moderate certainty)). Medical cannabis taken orally results in a very small improvement in physical functioning (4% modelled RD (0.1% to 8%) for achieving at least the MID of 10 points on the 100-point SF-36 physical functioning scale, WMD of 1.67 points (0.03 to 3.31, high certainty)), and a small improvement in sleep quality (6% modelled RD (2% to 9%) for achieving at least the MID of 1 cm on a 10 cm VAS, WMD of −0.35 cm (−0.55 to −0.14 cm, high certainty)). Medical cannabis taken orally does not improve emotional, role, or social functioning (high certainty). Moderate certainty evidence shows that medical cannabis taken orally probably results in a small increased risk of transient cognitive impairment (RD 2% (0.1% to 6%)), vomiting (RD 3% (0.4% to 6%)), drowsiness (RD 5% (2% to 8%)), impaired attention (RD 3% (1% to 8%)), and nausea (RD 5% (2% to 8%)), but not diarrhoea; while high certainty evidence shows greater increased risk of dizziness (RD 9% (5% to 14%)) for trials with <3 months follow-up versus RD 28% (18% to 43%) for trials with ≥3 months follow-up; interaction test P=0.003; moderate credibility of subgroup effect).

The authors concluded that moderate to high certainty evidence shows that non-inhaled medical cannabis or cannabinoids results in a small to very small improvement in pain relief, physical functioning, and sleep quality among patients with chronic pain, along with several transient adverse side effects, compared with placebo.

This is a high-quality review. Its findings will disappoint the many advocates of cannabis as a therapy for chronic pain management. The bottom line, I think, seems to be that cannabis works but the effect is not very powerful, while we have treatments for managing chronic pain that are both more effective and arguably less risky. So, its place in clinical routine is debatable.

PS

Cannabis is, of course, a herbal remedy and therefore belongs to so-called alternative medicine (SCAM). Yet, I am aware that the medical cannabis preparations used in most studies are based on single cannabinoids which makes them conventional medicines.

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