A cannabis plant extract provides pain relief for patients after major surgery such as knee replacements, a study by Imperial College London and the Medical Research Council has shown.

Details of a trial published today in Anesthesiology shows how effective Cannador, a cannabis plant extract, is at managing post-operative pain.

Dr Anita Holdcroft, from Imperial College London, and lead researcher 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 postoperative pain."

The researchers tested Cannador on 65 patients who had previously undergone surgery. 11 patients received a 5mg dose, 30 received a 10mg dose, and 24 received a 15mg dose. While all patients who received a 5mg dose requested additional pain relief, only 15 of those who received the 10mg dose and 6 of those on the 15mg dose did so.

As the dose increased, patients reported decreasing pain intensity and increasing side effects. Side effects included increasing nausea and increased heart rate in some patients.

Professor Mervyn Maze from Imperial College London, and one of the researchers, added: "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."

The study was conducted using patients from Chelsea and Westminster Hospital, Charing Cross Hospital, Northwick Park Hospital, Kings College Hospital, The Manor Walsall, The Whittington, St Bartholomew's, University College London Hospital, West Middlesex and Ravenscourt Park Hospital.

The study was funded by the Medical Research Council and the Westminster Medical School Research Trust, which is administered by the Chelsea and Westminster NHS Trust. The Cannador was donated by the Institute for Clinical Research, Berlin.