Smoking cannabis ‘worse than tobacco’

Smoking cannabis is more damaging than smoking tobacco, The Guardian reported. A single cannabis joint "may cause as much damage to the lungs as five chain-smoked cigarettes" it said on July 31 2007. Cannabis smokers were found to have damage to airways in the lungs that led to symptoms such as coughing, wheezing and chest tightness.

The researchers said the damage to the lungs is “likely to relate to the different characteristics of the cannabis joint and the way in which it is smoked”, with a hard inhalation, no use of filters and users holding the smoke in the lungs for a longer time.The study was well-conducted and found that cannabis use may be linked to adverse lung function, structure and symptoms. 

Where did the story come from?

The research was conducted by Sarah Aldington and colleagues at the Medical Research Institute of New Zealand, Wellington School of Medicine and Health Sciences, and Wakefield Hospital, all in Wellington, New Zealand. It was published in the peer-reviewed journal, Thorax .

What kind of scientific study was this?

This was a cross-sectional study to determine the association between smoking cannabis (with or without tobacco) and lung structure and function, and symptoms.

The study recruited 339 people, categorised into four groups: users of cannabis only; tobacco only; combined cannabis and tobacco; and non-smokers of either cannabis or tobacco.

People with chronic lung disease diagnosed before age 16 were excluded from the study, along with pregnant women and people with inherited enzyme deficiencies that may cause lung disease. Also excluded were people who had abused substances other than cannabis, tobacco, and alcohol more than 12 times in their lifetime.

Each participant undertook lung function tests, lung scans and completed detailed questionnaires relating to their lifetime use of cannabis, smoking history, passive smoking exposure, respiratory symptoms, family history, occupation and known respiratory illness.

What were the results of the study?

The researchers found a dose-response relationship between cannabis smoking and lung function. They found that one cannabis joint had a similar effect on airflow obstruction to smoking 2.5–5 tobacco cigarettes. Lung scans showed that cannabis smoking was linked with decreased lung density. Cannabis use was not linked to obvious signs of emphysema.

What interpretations did the researchers draw from these results?

The researchers concluded that smoking cannabis was linked to the impairment of large airways function, leading to airflow obstruction and hyperinflation, but there was no link to obvious signs of emphysema. They suggest that “the 1:2.5–5 dose equivalence between cannabis joints and tobacco cigarettes for adverse effects on lung function is of major public health significance”.

What does the NHS Knowledge Service make of this study?

This study was reasonably well-conducted, and indicates that cannabis use is likely to have adverse effects on lung structure, function and symptoms. However, in this type of study, when data is collected at one point in time, it is not possible to definitively conclude what causes the differences found. There are some limitations to the study, which are acknowledged by the authors:

• The study originally selected people using a random sample from the electoral register, but there were not enough cannabis smokers in this sample. It used advertisements to recruit people, which may have led to selection bias, by attracting people who were particularly concerned with respiratory health. 
• Many cannabis users were ineligible because of their abuse of other drugs, so the results only apply to a particular group of cannabis users. 
• People may not accurately remember their cannabis and tobacco smoking history, and it is difficult to quantify cannabis use precisely because of sharing of joints, different inhalation techniques and ways of smoking cannabis. 
• This was an exploratory study, which assessed a number of measures of pulmonary structure, function and symptoms, without correcting for multiple statistical tests, so it is possible that associations are due to chance rather than an actual difference in the population.

The dose response relationship, where higher intakes of cannabis were linked with poorer lung function adds strength to the demonstrated link shown in this observational study.