A cannabis plant extract provides pain relief for patients after major surgery, research has shown.
Cannabis drugs could help with pain after operations
An Imperial College London team tested the extract - Cannador - on 65 patients after surgery such as knee replacements and found it helped manage pain.
The researchers believe the results could lead to new pain relief drugs, even though the chance of side effects increased with stronger doses.
The research appears in the US journal Anesthesiology.
Lead researcher Dr Anita Holdcroft said: "Pain after surgery continues to be a problem because many of the commonly used drugs are either ineffective or have too many side effects.
"These results show that cannabinoids are effective and may lead to the development of a wider range of drugs to manage post-operative pain."
The researchers tested Cannador in different doses on 65 patients who had previously undergone surgery.
While all 11 patients who received a 5mg dose of the drug requested additional pain relief, only 15 of the 30 who received the 10mg dose and 6 of the 24 on the 15mg dose did so.
However as the dose increased some patients reported increased side effects such as nausea and increased heart rate.
Professor Mervyn Maze from Imperial College London, who also worked on the study, said: "We thought cannabis might be beneficial in helping manage pain following surgery, as previous research indicated cannabinoids help 'top up' the body's natural system for reducing pain sensation.
"This research proves it can be effective, with minimal side effects at low doses."
GW Pharmaceuticals, a Salisbury-based firm, is developing cannabis-based medicines under licence from the UK government.
One of its products, Sativex, has already been licensed in Canada for pain control in people with multiple sclerosis, and trials of the drug's ability to provide pain relief for people with advanced cancer are continuing.
Mark Rogerson, a company spokesman, said: "This latest trial is another welcome contribution to the body of evidence that cannabinoids have a role to play in medicine."