For years, people who would never dream of smoking a joint for pleasure have been secretly meeting drug dealers to top up their stash of cannabis.

They risk a criminal conviction because they say the drug relieves pain while conventional medicine fails.

Their claims, especially by people suffering from multiple sclerosis (MS), have received vocal backing from some corners of the medical world but some remain sceptical.

Now, research to discover if cannabis really works as a safe painkiller is to be carried out in Sussex.

The Queen Victoria in East Grinstead is one of 35 hospitals across the UK taking part in the research, measuring the effects of cannabis extract against other drugs for pain relief following an operation.

Doctors carrying out the study for the Medical Research Council are recruiting 400 patients who will be randomly assigned one of four oral pain-relieving treatments containing either standardised cannabis extract, tetrahydrocannabinol (an active ingredient in cannabis), a standard pain-relieving drug or a placebo.

The pain relief and side effects will be assessed over a six-hour period, with patients being questioned about the effects.

Anita Holdcroft, from Imperial College London, who is leading the £500,000 trial, said anecdotal evidence suggesting cannabis could provide effective pain relief for a variety of debilitating conditions needed to be assessed scientifically.

She said: "This is a proper study in a clinical setting where patients can be routinely monitored, using an oral capsule containing a prescribed dose.

"As not every volunteer is likely to be eligible, I'd really like to appeal to anyone who is asked to consider taking part to take the time to question the researchers, look at the information and see if they can contribute.

"No one will be left without access to pain relief during the trial, regardless of what treatment they are allocated and if successful, the research could help to improve patient choice."

If oral cannabis extract was found to be effective and without adverse side effects, it could provide another pain relief option to doctors and patients, such as those suffering from MS.

People taking part in the trial will need to be over 18 and cannot have had any exposure to cannabis or its constituents for six weeks prior to taking part.

Michelle Aylward, clinical nurse specialist for pain management at the hospital, said: "This is an opportunity to be involved in research that could have an impact on pain management in the future."

There are about 85,000 adults suffering from MS in the UK. Up to 10,000 are believed to use cannabis to relieve their symptoms.