Doctors have known of the medicinal benefits of cannabis for decades. Two drugs based on its active ingredient - tetrahydrocannabinol - have been used in the UK for over 30 years to treat nausea in cancer patients undergoing chemotherapy, although their use has declined as newer drugs have taken their place.

Interest in other medicinal uses of cannabis is growing. A 1997 report by the British Medical Association's Board of Science, "Therapeutic Uses of Cannabis", concluded that there was evidence that the drug could help muscle spasm in patients with multiple sclerosis. There was also limited evidence of benefits in epilepsy, glaucoma, asthma, high blood pressure and the weight loss associated with Aids.

The BMA stressed it was not advocating use of the whole cannabis plant, which could be as damaging as tobacco, but called on the courts to show compassion to people using it for medicinal reasons.

The aim of research is to extract the active constituents of cannabis, in the same way that morphine was purified from opium, in order to establish which are of benefit. There are at least 60 psychoactive substances among the 400 chemicals contained in the drug.

Two trials of the drug in patients were announced last year. A £1m Medical Research Council study is examining its effects in 660 patients with multiple sclerosis.

A second trial by the company GW Pharmaceuticals involving 2,000 patients suffering from MS, spinal cord injuries and intractable pain, is due to start this year.