News Release

Cannabis more damaging to health than previously thought claim doctors

Peer-Reviewed Publication

Imperial College London

Cannabis smoking could be responsible for up to 30,000 deaths in the UK, estimate doctors from Imperial College London and St Mary's Hospital.

Their editorial published in today's British Medical Journal, argues that cannabis could be a major contributor to deaths in the UK. The researchers calculate that if 120,000 deaths are caused among 13 million smokers, the corresponding figure among 3.2 million cannabis smokers would be 30,000, assuming that their health effects are the same.

Professor John Henry from Imperial College London, and one of the authors, comments: "Even if the number of deaths attributable to cannabis smoking turned out to be a fraction of the 30,000 we believe could be possible, cannabis smoking would still be described as a major health hazard. If we also add in the likely mental health burden to that of medical illnesses and premature death, the potential effects of cannabis cannot be ignored."

Although it has been well documented that cannabis use is associated with an increased incidence of mental disorders, it could have a major effect on the respiratory system.

Dr William Oldfield from St Mary's Hospital, and one of the editorial's authors, adds: "Cannabis and nicotine cigarettes have a very different mode of inhalation, with cannabis smokers taking a two thirds larger puff volume, a third larger inhalation, and holding the smoke down four times as long. These could all contribute to illnesses of the heart and respiratory system, particularly as the chemicals in cannabis smoke are retained to a much higher degree."

The level of active ingredient in cannabis, tetrahydrocannabinol (THC) has increased from around 0.5 per cent twenty years ago to almost 5 per cent today. THC also has marked effects on the heart and blood vessels, and sudden deaths have been attributed to cannabis smoking.

Dr Onn Min Kon from St Mary's Hospital, and one of the editorial's authors, adds: "At present there is still little evidence of cardiopulmonary harm from cannabis, and there are difficulties involved in gathering the data necessary. Despite the lack of evidence at the moment, this is still no reason for people to assume that cannabis is harmless, and cannabis users need to be fully aware of the risks."

Despite the fact that the number of tobacco smokers has been steadily decreasing due to public awareness of the harm caused, the numbers of cannabis smokers is increasing. Between 1999 and 2001, the number of 14 to 15 year olds who had tried cannabis rose from 19% to 29% in boys, and 18% to 25% in girls.


Notes to editors:

1. Can we compare cannabis with tobacco?, British Medical Journal, Volume: 326, Issue: 7396.
2. Consistently rated in the top three UK university institutions, Imperial College London is a world leading science-based university whose reputation for excellence in teaching and research attracts students (10,000) and staff (5,000) of the highest international quality. Innovative research at the College explores the interface between science, medicine, engineering and management and delivers practical solutions that enhance the quality of life and the environment - underpinned by a dynamic enterprise culture. Website:
3. St Mary's NHS Trust was named a three star NHS Trust in July, the highest possible Government performance rating. A teaching hospital, it has a comprehensive range of acute and specialist services delivered from two main sites: St Mary's Hospital in Paddington, and The Western Eye Hospital in Marylebone Road. It covers a 13 mile area including Westminster, Kensington & Chelsea, and South Brent, and incorporating more than 349,000 residents and employing over 3,000 staff.

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