Cannabinoids have been suggested to have therapeutic value as analgesics and in various conditions, including migraine headaches, nausea and vomiting, wasting syndrome and appetite stimulation in HIV-infected patients, muscle spasticity due to multiple sclerosis or spinal cord injury, movement disorders such as Parkinson’s disease, epilepsy, and glaucoma. When new therapeutic indications are suggested, two major factors should be taken into account: what are the adverse effects of the treatment and how does its effectiveness compare with that of existing alternatives?
In this week’s issue two high quality systematic reviews shed light on the therapeutic potential of cannabinoids in the management of pain and the nausea and vomiting induced by chemotherapy. Campbell et al sought and examined all randomised controlled trials that. compared the efficacy and safety of cannabinoids with those of conventional analgesics.   The nine trials included 222 patients, of whom 128 had cancer (five studies), two chronic non malignant pain (two studies, one patient per trial), and the rest postoperative pain. Cannabinoids were no more effective than codeine in controlling acute and chronic pain and they had undesirable effects in depressing the central nervous system. These studies are mostly from 1970s. Since then we have learnt to use non-steroidal anti-inflammatory analgesics alone and in combination with opioids in both cancer related and postoperative pain. There is thus no need for cannabinoids for these indications.
In chronic non cancer pain, however, we do need more effective analgesics than those currently available. Cannabinoids have anti-inflammatory effects, but it is difficult to believe that they would beat the anti-inflammatory drugs available today. Neuropathic pains, particularly those with spastic components are one area where cannabinoids may have potential. Future research may provide us with better cannabinoid compounds with potential new therapeutic applications. However, the current information is that the adverse effects of cannabinoids outweigh their effectiveness. About a year ago in the BMJ Strang et al asked for a more informed debate about the therapeutic use of cannabinoids, and this week’s two systematic reviews contribute to this debate. On current evidence cannabinoids can be recommended only for use in controlled clinical trials in carefully selected conditions for which there is no effective treatment. The launch of the first large multicentre trial on cannabis in the control of pain and tremors in multiple sclerosis is the first step on this way.