Medicinal cannabis does not cause serious side-effects when used to treat chronic pain, according to a new study.

When the drug is used as part of a treatment plan to help ease chronic pain, researchers have deemed it is 'reasonably safe'.

Scientists said they found no evidence of harmful effects on people's cognitive function, or blood tests, and they noted 'significant improvement' in their levels of pain, mood and quality of life.

Dr Mark Ware, from the Research Institute of the McGill University Health Centre in Montréal, said: 'This is the first and largest study of the long-term safety of medical cannabis use by patients suffering from chronic pain, ever conducted.

A new study by Canadian scientists has found daily cannabis use to help treat chronic pain does not cause any serious side-effects

'We found that medical cannabis, when used by patients who are experienced users, and as part of a monitored treatment programme for chronic pain over one year, appears to have a reasonable safety profile.'

Dr Ware's team found patients with chronic pain who used the drug daily for one year, under careful monitoring, did not have increased serious adverse effects compared to pain patients not taking the drug.

They say their findings, published online in The Journal of Pain, will serve as a benchmark study on the side effects of cannabis when the drug is used in pain management.

Researchers followed 215 adult patients with chronic non-cancer pain, who used medical cannabis.

They compared them to a control group of 216 chronic pain sufferers who did not use the drug.

The cannabis users were given access to herbal marijuana containing 12.5 per cent tetrahydrocannabinol (THC) - the active ingredient in the drug - from a licensed cannabis producer.

The drug was dispensed through the hospital pharmacy at each of the study locations, and patients collected their supply each month after completing necessary visits and tests.

Along with information on adverse effects, participants underwent lung function and cognitive testing.

And they were asked about their pain, mood and quality of life over the one-year follow-up period.

A number of the subjects underwent complete panels of blood tests for routine biochemistry, liver and kidney function, and selected hormone levels.

Scientists said they found no evidence of harmful effects on people's cognitive function, or blood tests, and they noted 'significant improvement' in their levels of pain, mood and quality of life

The average amount of cannabis used overall was 2.5 grams per day whether smoked, vaporised or taken as edibles.

Dr Aline Boulanger, director of the pain clinic at the Centre Hospitalier de l'Université de Montréal, said: 'Our data show that daily cannabis users had no greater risk than non-users to experience serious adverse events.

'We found no evidence of harmful effects on cognitive function, or blood tests among cannabis consumers and we observed a significant improvement in their levels of pain, symptom distress, mood and quality of life compared to controls. '

However, the researchers did report an increased risk of non-serious adverse events in medical cannabis consumers such as headache, nausea, dizziness, somnolence, and respiratory problems associated with smoking.

Dr Ware added: 'It is important to note the limitations of the study.

'Patients were self-selected, not randomised, and most were experienced users.

'So what we are seeing is that it appears to be a relatively safe drug when used by people who have already determined that it helps them.