Cannabis extract is to be trialled in a new South West study into Parkinson's Disease.
The trials based at Derriford Hospital will last for 12 weeks
The Peninsula Medical School is carrying out the study, which starts in May and will involve 24 patients from Cornwall and Devon.
The study will be based at Derriford Hospital in Plymouth and will look at whether the drug can help reduce the side effects of some existing medication.
Participants will be using a cannabis derivative called Cannador, which will be in capsule form.
Parkinson's is one of the most common neurological diseases in older people.
It is a progressive, degenerative, neurological condition for which there is currently no cure. Sufferers find increasing difficulty in moving their arms and legs.
They develop tremors and facial tics and gradually become more and more immobile. It is usually kept under control by tablets.
Although the tablets are effective in the short-term, in the long-term they can cause a condition known as dyskenesia - involuntary spasms of the limbs.
Researchers at Derriford Hospital believe Cannador could ease this.
Neurology registrar Dr Camille Carroll said: "A short study carried out in Manchester showed that a cannabis-type compound was helpful for reducing these side effects.
A study has been carried out into cannabis affecting multiple sclerosis
"It showed the abnormal movements, which can be quite disabling, were reduced by 30%.
"We're hoping we'll see a similar occurrence in this trial and this will be maintained in the tie of taking the capsules."
During the 12-week trial, the effects of the cannabis oil will be compared to a placebo.
If the study is successful, a case could be made for people with the condition to be prescribed Cannador on a more regular basis.
Dr Carroll said: "Parkinson's can be very debilitating. There is a spectrum and patients can range from being mildly affected to very severely affected.
"In the worst cases, patients are very rigid and stiff and find every movement difficult, so it's important that they have the treatment for it.
"But it's unfortunate that the treatment can be just as debilitating as the condition itself."
A report into a similar study carried out by the Peninsula Medical School into the effect of cannabis on patients with multiple sclerosis is due in the summer.