Oral cannabinoids have no major side effects and minimal to moderate effectiveness against the pain of musculoskeletal disease, a new meta-analysis finds.

The follow-up period was short, however.

"I think that for some people who fail other medications, or have contraindications, this could potentially be an option," Dr. Janet Pope, a co-author of the study at the University of Western Ontario in London, Canada, told Reuters Health by email. "But we don't know the long-term risk or benefit."

In a review of the Cochrane Library and other databases, Dr. Pope's team identified four randomized controlled trials involving 218 patients with rheumatoid arthritis, back pain or fibromyalgia.

On the 10-point Visual Analogue Scale for rating pain, patients on cannabinoids reported a mean difference of -1.47 compared to the control group. The research team classifies this effect as minimal to moderate.

Severe side effects were rare and the risk was statistically similar between the treatment and control groups. The studies, however, lacked the strength to draw firm conclusions on side effects, Dr. Pope and her team reported last week at the 2011 meeting of the European League Against Rheumatism in London.

There were no significant differences between groups in the risk of serious adverse events or adverse events that led to withdrawal.

In particular, Dr. Pope's team examined three side effects as secondary safety outcomes: drowsiness, confusion and euphoria. Two were significantly more common in the cannabinoid group: drowsiness (OR 4.05; number needed to harm: 4) and confusion (OR 5.48; number needed to harm: 9).

"The studies were short, so we don't know if the effect would be maintained or wear off over time, or if the side effects would change over time," Dr. Pope said.

The fact that such robust studies of cannabinoids exist is notable, Dr. Pope said. "There are randomized, controlled trials of oral cannabinoids for chronic pain but none of smoking marijuana in chronic pain."

Rob Goodier • Reuters Health Information © 2011