Cannabis might ease the pain

Marijuana may provide symptomatic relief of neuropathic pain associated with HIV, according to a new clinical trial. Participants who smoked cannabis cigarettes experienced greater pain reduction than those who smoked placebo cigarettes with cannabinoids (the psychoactive components of marijuana, including THC) removed.

The results should be interpreted with caution since just 28 HIV patients were examined, according to Prof Igor Grant, director of the Center for Medicinal Cannabis Research at UCSD, which funded the research. However, he continued to say that "converging evidence from several studies point in the same direction ... in the short-term smoked cannabis does have a beneficial effect on neuropathic pain".

Neuropathy is chronic pain due to nerve damage which, in HIV, is caused both by the illness and by common treatment. It affects 30% or more of HIV patients and "is a chronic and significant problem in HIV patients as there are few existing treatments that offer adequate pain management to sufferers", said Prof Ronald Ellis, who led the study.

The trial, conducted at the University of California, San Diego School of Medicine, examined HIV patients whose pain had not responded well to other drugs, opiates included. Since the patients continued taking their conventional medicines, Grant said the study can only tell researchers about the additional (or adjunctive) effect of cannabis on neuropathic pain.

The patients were randomly selected to smoke cannabis or placebo cigarettes over five day periods and the outcome was assessed used standardised tests of pain relief experienced by the patients. To guard against bias, the study used a double-blind approach where neither the patients nor the experimenters knew whether any given patient was smoking cannabis or placebo cigarettes. However, most of the patients taking cannabis guessed that they were not taking a placebo.

Published in the journal Neuropsychopharmacology, the trial showed that 46% of the cannabis smokers reported clinically meaningful pain relief (30% reduction) as compared to 18% of the placebo smokers.

Cannabionids relieve pain by acting on pain receptors in the human nervous system where they "mimic the effect of endocannabinoids which are released in a protective manner" to combat pain, said Roger Pertwee, professor of Neuropharmacology at the University of Aberdeen. According to Grant, the UCSD researchers are interested in smoking cannabis since THC taken orally is irregularly absorbed into the bloodstream.

The relatively small scale of the trial "would be enough for a scientific study but drug licensing bodies would require many more", said Pertwee who is also Director of Pharmacology at GW Pharmaceuticals which produces Sativex, a cannabinoid drug for pain relief in multiple sclerosis and certain cancer patients. Sativex is approved in Canada but is not licensed in the UK.