Cannabis prescriptions could soon be available as evidence mounts for its medicinal use for people with cancer and multiple sclerosis.
Australian doctors are testing a cannabis mouth spray called Sativex in cancer patients with pain that does not respond well to traditional painkillers such as morphine.
The phase three trial is the last of several studies required for manufacturer GW Pharmaceuticals to try to license the drug in Australia.
Dr Brian Le, a palliative care specialist at the Royal Melbourne Hospital, said four Australian hospitals were participating in the study which involves about 300 patients in 20 locations around the world. If the drug is found to be safe and effective, it could be available to Australian patients in the next few years.
''This is a really good study and one that's quite promising,'' he said.
A spokeswoman for Novartis, the company employed by GW Pharmaceuticals to commercialise the drug in Australia, said it was also working with the Therapeutic Goods Administration to make Sativex available to patients with multiple sclerosis who suffer from uncontrolled muscle spasms.
No pharmaceutical drugs based on cannabis are currently licensed for use in Australia. Since 2004, health authorities in Britain, Canada and Spain have licensed Sativex for patients with MS, a neurological condition that causes painful muscle stiffness in about 90 per cent of sufferers.
While cannabis is known to cause hallucinations and delusions, Dr Le said phase one and two trials of Sativex in more than 400 cancer patients found it relieved pain with few side effects, the most common being nausea. He said psychiatric side effects were very rare in the doses used and that patients did not report feeling ''stoned''.
''Our experience is that pain improves and the patients actually feel better. They're more able to do daily activities, sleep better through the night without pain and think clearly, so instances of feeling confused or out of it are low,'' he said. ''They don't feel better because they're stoned, they feel better because their pain is well controlled.''
Dr Le said the drug worked by targeting cannabinoid receptors in the brain. ''They mediate how the pain is sensed and how that pain message is transmitted to the brain and therefore perceived, so it reduces the conduction of that message to say there is pain occurring.''
Dr Le said new painkillers were needed for cancer patients because about 20 per cent of the 80 per cent who experienced pain could not be treated adequately. ''Pain is all pervasive. Having a cancer diagnosis in itself can be devastating, but having pain on top of that constantly reminds you that you're unwell,'' he said. ''It constantly interferes with what you can do and where your life is headed so being able to control that without too many adverse effects is great.''
The director of the National Cannabis Prevention and Information Centre, Professor Jan Copeland, said low doses of Sativex were unlikely to have the same effect on people as illicit cannabis because it was made of almost equal parts of tetrahydrocannabinol (THC), the active chemical in cannabis that most recreational users look for, and cannabidiol (CBD), the ''good cannabis'' that typically lowers anxiety and psychotic symptoms.
''These days, cannabis is bred to have high THC and no CBD so that may be why we're seeing adverse effects on mental health,'' she said. ''So Sativex is not like the cannabis smoked on the streets.''
Professor Copeland is studying whether Sativex can help drug-dependent people withdraw from cannabis. The results of a trial involving 51 people will be reported next year.
Surveys show about 1 per cent of the Australian population uses illicit cannabis, and 200,000 use it daily.