A painkiller containing THC, the active ingredient in cannabis, is used by 1,400 MS sufferers

A cannabis-based painkiller has performed little better than a dummy drug in trials, raising doubts about its effectiveness in treating multiple sclerosis.

Sativex, which contains THC, the active ingredient in cannabis, is being used as a painkiller by around 1,400 MS sufferers in Britain.

The mouth spray's makers, Wiltshire-based GW Pharmaceuticals, also hope to market it to ease the muscle spasms associated with MS and the pain of cancer.

However, its worth has been thrown into doubt by a test involving more than 300 MS patients.

While half of those taking Sativex found their pain was reduced by at least 30 per cent, those using a dummy spray fared almost as well.

GW said the result could be explained by an "unexpectedly strong placebo response" - in which symptoms are eased by the simple belief that a treatment will work.

The news sent the company's shares plummeting by as much as 31 per cent. GW, however, remains adamant there was a place for Sativex on the pharmacist's shelf.

Managing director Justin Gover said there was a ' desperate need' for new pain treatments and the drug had a "real role" to play.

The treatment, which is sprayed under the tongue four or five times a day, first went on the market in Canada in 2005.

Britain's drug regulators have yet to deem it safe and effective. However, doctors can prescribe it on a 'named patient' basis, which means they take personal responsibility for giving the drug.

A typical daily treatment of five sprays costs around Ł4.

GW grows 40,000 cannabis plants a year at a secret location in the English countryside.

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