Patients are being recruited for a trial to determine whether chemicals in cannabis can slow the impact of multiple sclerosis.
Derivatives of cannabis are known as cannabinoids
Evidence suggests the drug may relieve symptoms but the three-year national trial is also to determine whether it slows the disease's progress.
It is estimated that 85,000 people in the UK have multiple sclerosis (MS).
Prof John Zajicek, of the Peninsula Medical School and Derriford Hospital in Plymouth, will lead the research.
One component of cannabis, called THC, is now being tested in a trial, funded by a £2m grant from the Medical Research Council, along with charities the MS Society and MS Trust.
"This trial will build on our previous study which, coupled with our work in the laboratory, suggested that THC could have a protective effect on nerves," said Prof Zajicek.
"Multiple Sclerosis is a very unpredictable disease. Currently there are few medicines which are effective in treating MS and none have been shown to have any effect in the progressive stages of the disease."
MS is caused when the patient's own body damages the protective covering of the nerves - affecting signals from the brain.
Progressive MS is thought to be caused by damage to the nerves themselves.
"If this study demonstrates that cannaboids do have a longer term effect on the progression of disability, there are potentially far-reaching implications, not only for the health of people with MS, but also for those with other neurodegenerative conditions."
Prof Zajicek is trying to recruit 500 patients with progressive MS through 30 centres across the UK.
The research follows on from a previous trial carried out by the same team, which focused on testing the benefit of cannabis derivatives over a 15-week and 12-month period.
Derivatives of cannabis are known as cannabinoids.
The study is taking place in collaboration with Professor Alan Thompson at the National Hospital for Neurology and Neurosurgery (part of University College London Hospitals NHS Foundation Trust) and Institute of Neurology, University College London.
Potential patients interested in taking part in the study should call 0800 0153430, or e-mail: firstname.lastname@example.org