A drug made from an extract of cannabis has helped to reduce the pain caused by rheumatoid arthritis.
Cannabis is being grown under licence
The drug, Sativex, has been developed by GW Pharmaceuticals, which is assessing the medical benefits of cannabis under a UK government licence.
Tests of a spray form of the drug on 58 arthritis patients showed it helped reduce pain, and improve quality of sleep.
Few people showed signs of side effects, the company said.
GW Pharmaceuticals has previously carried out trials showing that Sativex can reduce the pain associated with multiple sclerosis.
Dr Philip Robson, director of GW's Cannabinoid Research Institute, said: "These results are particularly exciting because this is the first ever controlled clinical trial of a cannabis-based medicine in the treatment of arthritis.
"To date, GW's research has concentrated on multiple sclerosis and neuropathic pain and it is therefore very encouraging to see these positive effects of Sativex on pain and other symptoms of rheumatoid arthritis.
"This exploratory trial provides further strong support to our belief that cannabis-based medicines may offer therapeutic potential across a range of medical conditions."
The research will now focus on the most effective dose to give patients.
The study was welcomed by the Arthritis Research Campaign.
A spokeswoman said: "It's not going to cure the disease, but it will do a lot to alleviate the pain and suffering of people with rheumatoid arthritis.
"Cannabis is probably less harmful than other available painkillers.
"This idea that people with rheumatoid arthritis will be sitting around smoking joints and getting high is quite wrong; cannabis-based pain killers should be taken very seriously."
Arthritis Research Campaign scientists have previously carried out studies which showed that cannabidiol - a natural constituent of cannabis that has no mind-altering effects in its purified form - can ease the effects of collagen-induced arthritis in mice.
GW cultivates some 40,000 cannabis plants a year at a secret location in the English countryside.
The government has already said it would grant permission for the use of cannabis-based medications if trials produced positive results.