Cannabis, also known as marijuana, is not a new drug, having been used for various medical, religious and recreational purposes for over 4000 years. It has been increasingly used over the last 50 years to the extent that it is now the most widely used illegal drug worldwide. New Zealand has one of the highest reported rates of cannabis use, with about three-quarters of New Zealanders having tried cannabis by the age of 25, and nearly 10% cannabis-dependent by this age.
The potential adverse effects such as the dependence syndrome, impaired adolescent psychosocial development and mental health, and increased risk of motor vehicle crashes are well recognised. In contrast the potential for habitual cannabis smoking to cause adverse respiratory effects is less well recognised, in part because there has been less research undertaken in this field. In some respects this is surprising, as the usual way to take cannabis is by smoking, and it is known that cannabis smoke contains many of the same constituents as tobacco smoke, including higher levels of some carcinogens. However, over the last 10 years there has been a greater research effort to better understand the respiratory effects of habitual cannabis smoking, including a number of studies from New Zealand, which have contributed to knowledge in this field. In this brief review, eight key observations from New Zealand studies of the respiratory effects of cannabis are presented.
- Significant respiratory symptoms and impaired lung function occur in cannabis-dependent individuals by the age of 21 years, after controlling for tobacco use, even though the cannabis smoking history is of a relatively short duration.
- Smoking cannabis in addition to tobacco has an additive effect in causing some respiratory symptoms and impaired lung function.
- Whereas cannabis smokers use similar amounts of cannabis, regardless of whether they smoke tobacco as well, tobacco smokers who use cannabis smoke less tobacco than those who smoke only tobacco. However, this lesser amount of tobacco in combined users does not result in reduced respiratory effects, compared with tobacco only smokers, due to the additional effects of the cannabis smoking.
- Habitual cannabis smoking is uncommonly associated with emphysema, which occurs almost entirely in heavy cigarette smokers.
- One cannabis joint is equivalent to between 2.5 and 5 tobacco cigarettes for adverse effects on lung function. This dose equivalence is consistent with the reported 3- to 5-fold higher levels of carboxyhaemoglobin and tar inhaled when smoking a cannabis joint compared with a tobacco cigarette of the same size.
- Long term cannabis smoking increases the risk of lung cancer in young adults. The magnitude of the risk can be viewed in different ways:
- for each joint-year of cannabis smoking (1 joint-year = smoking 1 joint per day for 1 year, or 1 joint per week for 7 years, or equivalent), the risk of lung cancer increases by 8%.
- the population attributable risk of lung cancer with cannabis smoking in young adults is about 5%, i.e. cannabis smoking causes one in every 20 cases of lung cancer in young adults.
- One cannabis joint is similar to 20 tobacco cigarettes in terms of lung cancer risk. While this is a crude estimate, it is consistent with the observation that smoking ‘a few’ cannabis joints a day causes similar abnormal cellular changes in the airways as smoking 20 to 30 tobacco cigarettes.
- There is weak evidence that starting cannabis smoking at an earlier age leads to a greater risk of lung cancer. Such an association would be consistent with the observation that the younger a person starts smoking tobacco, the greater the lung cancer risk.
So what are the public health implications of these research findings? It seems reasonable to conclude that public health programmes which are targeted to reduce harm from tobacco smoking need to include greater initiatives to reduce cannabis smoking, and should be directed particularly at younger people. While the major concerns regarding cannabis smoking are appropriately focussed on the mental health effects, it is important that the public is also informed about the potential respiratory effects, so they can make an informed choice, as with tobacco smoking. Finally, mention should be made of the generous participation of cannabis smokers in the New Zealand research studies, and the funding through the Health Research Council of New Zealand, which have led to this knowledge of the respiratory effects of cannabis smoking.
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