Cannabis pills will be given to 400 NHS patients in an experiment to see if the drug alleviates pain after surgery.

The patients will be split into four groups at 36 hospitals. Some will take a pill containing cannabis extract, others a pill containing the drug's active ingredient, tetrahydrocannabinol, and the rest a painkiller or a placebo. The patients will be given the pills after surgery once they have stopped using morphine and will be monitored over six hours.

Dr Anita Holdcroft from Imperial College London, who is running the £500,000 trial from the Chelsea and Westminster Hospital, said anecdotal evidence suggested cannabis could relieve pain but doctors needed to assess the evidence scientifically and compare it with existing medications.

The trial, funded by the Medical Research Council (MRC), was launched earlier this year but difficulties in recruiting volunteers prompted Dr Holdcroft to appeal for people to come forward yesterday.

Medicines derived from the cannabis plant are being tested by drug companies around the world as treatments for stiffness, tremor, weak bladder, loss of appetite and high blood pressure, as well as pain.

Positive findings from a £1.2m trial of cannabis in patients with multiple sclerosis, also funded by the MRC, are expected later this month. The study, the largest in the world, is expected to show cannabis helps to reduce muscle spasms and improve bladder control.