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Medical Marijuana for Pain: What the Evidence Shows

Medical Marijuana for Pain: What the Evidence Shows

© Lightspring/shutterstock.com, Medical Marijuana for Pain© Lightspring/shutterstock.com
Few subjects in medicine are the focus of greater attention than the medical use of marijuana. Twenty-three states and the District of Columbia have now approved forms of cannabinoids for various health problems. The rules and regulations regarding use vary from state to state, but in virtually all, chronic pain is included as one of the conditions for which cannabis can be prescribed.

If one listens to advocates of medical marijuana, it seems like a miracle drug that not only is far more effective than the currently available analgesics but also has a more benign adverse-effect profile. Two new reviews of studies provide a clearer picture as to just how close these claims fit with what actual research has shown.

The first is a review and meta-analysis of cannabinoids for multiple medical conditions, including pain.1 Of the 79 trials for all conditions the review identifies, only 4 studies were found to have a low risk of bias based on the methodology employed; 55 studies were found to have high risk; and the risk was unclear in 20 studies. Although most of the studies indicated they were double-blind, only 57% were found to actually have employed methods to ensure this with study participants and only 24% were found to have appropriately blinded the outcome assessors.

Twenty-eight studies were undertaken to evaluate the effectiveness of cannabinoids for chronic pain. Of these, only 2 were found to be at low risk of bias. The most commonly used cannabinoids were nabiximols—used in 13 studies; the rest of the studies evaluated the effect of smoked tetrahydrocannabinol (THC), nabilone, THC oromucosal spray, and dronabinol. One study compared cannabinoids with an active ingredient, the analgesic amitriptyline; in the others, cannabinoids were compared with placebo.

In 17 of the studies, the most common form of pain was neuropathic pain, including central pain, diabetic peripheral neuropathic pain, and HIV-associated neuropathy; 3 or fewer studies included cancer pain, fibromyalgia, arthritic pain, and a variety of other pain conditions.

Across the studies, more patients reported a 30% or greater reduction in pain with the cannabinoids than with placebo. On the basis of these studies, the authors of the review rated the support for cannabinoid use as moderate. Smoked THC appeared to provide the most benefit.

The second review examined 5 randomized controlled studies on cannabinoids for neuropathic pain and 6 for other forms of pain.2 All but one of the studies found a significant decrease in pain with cannabinoids compared with placebo. However, in the crossover study that compared nabilone with the opioid dihydrocodeine, the latter provided better analgesia.

Overall, these studies seem to indicate that cannabinoids have a significant role to play in the management of chronic pain. However, there are important issues that limit the validity of this conclusion. First and most important is how the improvement in pain was evaluated. In many of the studies, only instruments to measure the level of pain, most notably the Visual Analogue Scale, were used. This is fine when one is measuring acute pain. But when it comes to chronic pain—which is what the studies were looking at—the most important measures of the impact of any treatment are improvement in functioning and other objective measures, such as reduction in use of analgesic medications.

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