While the medical use of cannabis has expanded, there's little data available regarding its safety. Although the drug has been used (recreationally and medically) by humans going back far into prehistory, it was criminalized by the time researchers began conducting rigorous clinical trials. Consequently, almost every news story one reads about the use of cannabis as a medical therapy contains some variation of disclaimer saying "more research is needed" into the longterm safety of medical cannabis use.

Now a tiny bit of that "more research" has been published in the Journal of Pain. The headline result was that there was no increase in the number of serious adverse events in a group that used cannabis for chronic pain when compared to a group that did not. As the authors point out in the paper, the "lack of data on the safety and efficacy of cannabis is a major barrier to physicians’ involvement [in prescribing medical cannabis]."

The study was conducted in Canada between 2004 and 2008. It followed 431 chronic pain patients for a year in order to assess the rates of adverse events, pulmonary effects, and neurocognitive function. The patients were divided into a group that used cannabis to treat that chronic pain (n=215) as well a control group that didn't (n=216). A key strength of the work is that it was a prospective study; the participants were chosen before they started the treatment plan.

As noted above, there was no difference in adverse effects in the cannabis-using group—in and of itself, that's quite a useful finding. The study also found no significant change in neurocognitive function between cannabis users and control. That finding contradicts studies on recreational users that have found a negative effect on both attention and memory with longterm cannabis use.

Cannabis users also had significantly larger improvements in pain intensity and quality of life compared to the control group. So the authors concluded that doses of 2.5g/day (0.09 ounces for those of you keeping track) can be considered safe for treating chronic pain patients as part of a "carefully managed pain program."

However, there are some caveats. First, although the rate of serious adverse events was not significantly different between the cannabis and control groups, cannabis users did report a larger number of non-serious adverse events. These were mainly neurological, respiratory, infectious, and psychiatric events, and all were mostly mild or moderate in nature.

Next, the study had relatively few participants, which limits the power of the statistical analyses, plus around 30 percent of participants dropped out before the 12-month followup. Following participants for longer periods and looking at more naive users (those who hadn't used cannabis before being prescribed it for chronic pain) would both be welcome additions to future studies.

Still, the authors recognize all of these caveats in their work, suggesting that further studies ought to be conducted to address these issues. As cannabis laws are eased in the US and elsewhere—both for medicinal and recreational use—it ought to be possible to conduct larger trials over longer timeframes.

J. Pain 2015. DOI: 10.1016/j.jpain.2015.07.014 (About DOIs).”