Last updated January 2015
Dr Peter Gates, NCPIC
This bulletin is about the link between asthma and smoking cannabis. As with tobacco smoke, cannabis smoke contains harmful levels of carcinogenic compounds including benzopyrene, benzanthracene, phenols, vinyl chlorides, nitrosamines and reactive oxygen species.
- More than a third (35.4%) of Australians have smoked cannabis and more than 1 in 10 have smoked it in the past year.
- Those currently aged 14-19 years are more likely to have smoked cannabis than tobacco. People with asthma are not less likely to report smoking cannabis or tobacco than those who do not have asthma.2-12 This is a concern, as smoking cannabis can cause more asthma symptoms and more frequent asthma attacks.2, 13-17 Among young people with asthma, those who have reported ever smoking cannabis before the age of 13 years are approximately twice as likely to have had an asthma attack in the past year.2
As with tobacco smoke, cannabis smoke (as a product of combustion from a pipe, joint or bong) contains harmful chemicals which can damage the lungs, and stop them from working properly.18-20 For some people, this damage is increased due to a prolonged and deeper inhalation when smoking cannabis compared with tobacco. This style of smoking can lead to a three-fold increase in the amount of tar and nearly five-fold increase in the carbon monoxide that is taken into the lung.21
People who smoke cannabis often have more coughing and wheezing, and produce more sputum, compared to non-smokers.22-28 This becomes more likely and more severe as the exposure to cannabis smoking increases.17 In addition, cannabis smokers develop a similar type of inflammation in the airways as tobacco smokers.29-32 This may reduce the effectiveness of asthma preventer medications and may lead to higher doses being required to achieve good asthma control.
Importantly, these negative effects on the respiratory system are made even worse when smoking cannabis and tobacco together.24, 33, 34
Surprisingly, cannabis was once advertised as a treatment for asthma before the development of modern medicine.35 This may have been because of a reduction in constriction of the airways (bronchospasm) and an initial opening of the airway immediately following use.16, 36-43 These effects of cannabis smoking are thought to be modest and smoking cannabis is likely to be less effective than using bronchodilators such as salbutamol or terbutaline (Ventolin, Asmol, Bricanyl).44 The negative effects on respiratory health from cannabis smoking are more pronounced in the medium and long term.
Smoking cannabis in the medium to long term has two main negative effects on the airways:
1. Increases and damages the mucus lining in the airways
- more mucus-producing cells and glands grow in the airway walls22, 24, 26-28
- the mucus which is produced may be damaged and immune cells which respond to the ingestion of toxic agents may be less effective31, 45-49
2. Damages the airways
- causes inflammation in the airways29-32
- some research has shown that chemicals in cannabis smoke can result in a narrowing of the airway following long-term, regular use41, 50
Some people with asthma believe that by not smoking cannabis, but rather vaporising the plant, they will avoid the harmful effects from cannabis smoke. While this method is likely to reduce some of the negative impact on respiratory health, it is not without harm as toxic levels of the potent and long-lasting neurotoxin ammonia are produced from heating cannabis in this way.51 Moreover, there is no long-term research on the impact of using a vaporiser and other alternative means of using the drug.33
Unfortunately there is also very little research on the impact of cannabis smoke when inhaled passively. Given cannabis smoke can contain even higher levels of carcinogens than tobacco smoke the same precautions should be taken to reduce passive exposure to children and pets18. Extra precaution should also be taken around individuals with asthma and other respiratory and cardiovascular diseases.
Finally, there is mixed evidence that cannabis smoking is associated with an increased risk of suffering a collapsed lung (pneumothorax).44, 52 As there have been several cases of lung collapse that may be due to unusual inhalation techniques, this possibility is of concern.53-60
Other health issues
Smoking cannabis is associated with many other health issues including an increased risk of developing cardiovascular disease as well as mental health concerns such as addiction, psychosis and schizophrenia.61 As asthma is also associated with mental health concerns such as depression62, the interaction of these comorbidities deserves exploration.
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