Marijuana smoke is similar to tobacco smoke in several ways. Marijuana smoke has about the same amounts of tar and more cancer-causing chemicals than tobacco smoke.1 Both types of smoke increase inflammation.2,3 Both trigger symptoms such as cough and wheeze.1,3 However, marijuana smoke may affect airway obstruction differently.2

Experiments in the 1970s showed that smoking marijuana opens up the airways. In contrast, tobacco smoke causes the airways to narrow.3 In one study, eight people with asthma did methacholine and exercise challenge tests.4 After the challenges, they were treated with a short-acting beta-agonist, marijuana, placebo marijuana, and saline. Treatment with marijuana or the short-acting beta agonist opens up the airways immediately.

This finding led to some studies of marijuana and lung function. Nevertheless, neither of the major guidelines recommend—or even mention—marijuana as a treatment for asthma.5,6 In most of the United States, marijuana use is illegal or restricted to certain medical conditions. The list of approved medical conditions varies from state to state.7

How does marijuana affect lung function?

As the early studies showed, marijuana opens up the airways.4 This effect lasts for about one hour.8

Another study showed that people who smoke marijuana have more lung capacity.2 This study included people with and without asthma. One theory to explain this is that people smoke marijuana differently than cigarettes. They tend to inhale marijuana more deeply and hold their breath longer.1 This action might stretch the lungs or strengthen the chest wall muscles.8 These changes may affect the results of lung function tests. However, it is not clear how such changes might affect lung function in daily life.8

One study included more than 5000 people with and without asthma.8 Information about tobacco use, marijuana smoking, and lung function was collected for 20 years. People who used tobacco had less lung function. On the other hand, low-level marijuana was not linked to lower lung function. Low-level marijuana use was defined as 7 joint-years or less. (Joint-year is calculated as joints smoked per day times the number of years of marijuana use.) This study did suggest that heavy marijuana use might reduce lung function.

Marijuana and tobacco smoking often overlaps. One study showed that 77% of people who smoked marijuana also smoked tobacco.1 This makes it hard to study their different effects.

What are the asthma-related risks of smoking marijuana?

Marijuana smoke is generally unfiltered. Smokers inhale marijuana more deeply than tobacco smoke. As a result, more tar ends up in the lungs.1 Marijuana smoke damages the lining of the airway and increases airway inflammation.8

Compared with nonsmokers, marijuana smokers are more likely to have bronchitis, cough on most days, phlegm, wheezing, and chest sounds.1 Their breathing symptoms are very similar to tobacco smokers.

What are other risks of smoking marijuana?

Risks of smoking marijuana include:9

  • Problems with short-term memory.
  • Making bad decisions.
  • Increased anxiety or paranoia.
  • Decreased coordination and reaction time.

Marijuana can be harmful for children and teens.9 Their brains are still developing. Prenatal marijuana exposure can cause problems with a baby’s nervous system. Children exposed to marijuana before birth do not do as well in school. There is a risk of addiction to marijuana.9

Written by: Sarah O'Brien | Last Reviewed: May 2016.
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