There have long been anecdotal reports that marijuana can have pain-killing (analgesic) effects in types of chronic pain which are otherwise difficult to treat. This has led to great enthusiasm about the prospect of "medical marijuana" - but, attractive as that might sound, there has always been a lack of hard evidence showing that marijuana in fact works. Being highly illegal in the U.S.A (more illegal than cocaine in fact), it's hard to study.
A paper out today in Neuropsychopharmacology aimed to test the analgesic power of smoked marijuana vs. placebo and found that it was moderately effective. The work was done at the University of California (where else?) San Diego, and the subjects were all men with HIV. Like many such patients, they all suffered from chronic pain due to nerve cell damage, caused by either the virus itself or certain anti-HIV drugs. The patients in this study had all been diagnosed with "distal sensory predominant polyneuropathy" (DSPN), and were already taking standard painkillers such as aspirin and/or opiates such as morphine. This was pain that wouldn't go away.
30 volunteers were randomly assigned to smoke four times per day for 5 days; they smoked either marijuana cigarettes or placebo cigarettes which were similar in appearance but which lacked the active chemicals of marijana. After a two week break the subjects got the other kind for cigarette for five days (a cross-over design). Most, but not all, of the patients had some experience with (illegal) cannabis previously.
The results - After smoking marijuana, the patients rated their pain as being significantly less than when they were on placebo. The difference was significant although not enormous at first glance - a median difference in pain reduction of 3.3 "DDS points" (starting score median was 11.1) which translated into a standardized effect size of 0.60 ("medium" to "large"). So marijuana didn't completely kill the pain, but it was a decent help, and the effect was comparable to that seen with other drugs used in neuropathic pain such as anti-convulsants.
There was one snag, however. Although the side effects of the cannabis were generally minor, one of the patients, who had never tried cannabis before, developed "cannabis-related psychosis" on the first day they smoked the real cannabis.
One cannabis-naive subject had an acute, cannabis-induced psychosis at the start of the second smoking week; unblinding revealed that he had received placebo during the first week and active cannabis during the second.
Marijuana use has been correlated with an increased risk of psychosis. Whether the link is a causal one is hotly debated, but it's plausible that it could be, so it's something to bear in mind. The problem is that, frustratingly, the authors don't tell us anything about what "an acute, cannabis-induced psychosis" actually means. Is this just their melodramatic way of describing the unpleasant but fairly harmless experience of "pulling a whitey"? How long did it last? Did the patient require psychiatric treatment? Full-blown psychosis from the first dose of cannabis sounds rather implausible. Also, a number of drugs currently on the market can cause psychosis in rare cases, such as L-dopa - it shouldn't be seen as the end of the world for a drug, especially if it's one which fills an important niche.
So, this is evidence that marijuana can be helpful in one form of chronic neuropathic pain. Although it was a small, short study in a fairly narrowly defined group of patients, this is a believable result, given the anecdotal evidence and given the fact that mountains of animal studies show that marijuana-like drugs (CB1 receptor agonists) are analgesic in animals. It's a cliché that medical research papers always end with a call for further study on the question at hand, but in this case, I think that really is warranted. If marijuana can help treat intractable chronic pain, we need to know about it, and that means we need more randomized controlled trials.
Ronald J Ellis, Will Toperoff, Florin Vaida, Geoffrey van den Brande, James Gonzales, Ben Gouaux, Heather Bentley, J Hampton Atkinson (2008). Smoked Medicinal Cannabis for Neuropathic Pain in HIV: A Randomized, Crossover Clinical Trial Neuropsychopharmacology, 34 (3), 672-680 DOI: 10.1038/npp.2008.120