The results contradict findings from the first phase of the study, where improvements seemed to be largely due to "good moods".

“There does seem to be evidence of some benefit from cannabis in the longer term that we didn’t anticipate in the short term study,” says John Zajicek, at Peninsula Medical School in Exeter, UK, and one of the research team.

In 2003, Zajicek and his colleagues published results on the largest study to date of cannabinoids and MS. The trial included 630 advanced-stage MS patients who took either cannabinoid compounds or a placebo for 15 weeks.

Compared with those on placebos, patients who received active compounds said they both felt less pain and less muscle spasticity – the spasms characteristic of this neurodegenerative disease.

Good guess

But physiotherapists using standard evaluations were unable to corroborate the patients' claims of improved mobility or muscle stiffness.

The results were further complicated because about two thirds of the patients who received cannabis compounds, such as D9-tetrahydrocannabinol (THC), guessed they had not received a placebo, due to the drugs effect on their mind.