Marijuana smoke contains a similar range of harmful chemicals to that of tobacco smoke (including bronchial irritants, tumor promoters and carcinogens) (Hoffmann et al, 1975). As inhaled smoke comes into contact with airway and lung before being absorbed into the bloodstream, it is likely to affect the respiratory system (Novotny et al, 1982).

Risk of respiratory effects from inhaling marijuana smoke are heightened by the more intensive way in which marijuana is smoked — when smoking marijuana compared to tobacco, there is a prolonged and deeper inhalation and it is smoked to a shorter butt length and at a higher combustion temperature. This results in approximately 5 times the carbon monoxide concentration, 3 times the tar, and the retention of one-third more tar in the respiratory tract. Higher levels of ammonia and hydrogen cyanide have also been found in marijuana smoke, compared to tobacco (Moir, et al., 2008; Wu et al., 1988; Tashkin et al., 1991; Benson & Bentley, 1995).

A 2011 systematic review of the research concluded that long-term marijuana smoking is associated with an increased risk of some respiratory problems, including an increase in cough, sputum production, airway inflammation, and wheeze – similar to that of tobacco smoking (Howden & Naughton, 2011). However, no consistent association has been found between marijuana smoking and measures of airway dysfunction. Occasional and low cumulative marijuana use has not been associated with adverse effects on pulmonary function (Pletcher et al., 2012); the effects of heavier use are less clear.

Additionally, many marijuana smokers also smoke tobacco, which further increases the harm. Numerous studies have found that the harmful effects of smoking marijuana and tobacco appear to be additive, with more respiratory problems in those who smoke both substances than in those who only smoke one or the other (Wu et al, 1988).

The association between smoking marijuana and lung cancer remains unclear.  Marijuana smoke contains about 50% more benzopyrene and nearly 75% more benzanthracene, both known carcinogens, than a comparable quantity of unfiltered tobacco smoke (Tashkin, 2013). Moreover, the deeper inhalations and longer breath-holding of marijuana smokers result in greater exposure of the lung to the tar and carcinogens in the smoke. Lung biopsies from habitual marijuana-only users have revealed widespread alterations to the tissue, some of which are recognized as precursors to the subsequent development of cancer (Tashkin, 2013).  

On the other hand, several well-designed and large-scale studies, including one in Washington State (Rosenblatt et al, 2004), have failed to find any increased risk of lung or upper airway cancer in people who have smoked marijuana (Mehra et al, 2006; Tashkin, 2013), and studies assessing the association between marijuana use and cancer risk have many limitations, including concomitant tobacco use and the relatively small number of long-term heavy users – particularly older users. Therefore, even though population-based studies have generally failed to show increased cancer risk, no study has definitively ruled out the possibility that some individuals, especially heavier marijuana users, may incur an elevated risk of cancer. This risk appears to be smaller than for tobacco, yet is important to consider when weighing the benefits and risks of smoking marijuana. (Tashkin DP, 2013). More research on marijuana smoking and cancer is needed.

Two other conditions of concern, bullous lung disease (abnormal airspaces in the lungs caused by damage to the lung walls) and pneumothorax (“collapsed lung”), have not been definitively linked to marijuana smoke either (Tam et al, 2006). Several studies have found evidence of a possible association (Beshay et al, 2007; Hii et al, 2008; Reece, 2008), however, many of these studies featured 10 or fewer study subjects, some of whom also smoked tobacco. The research remains unclear.

Respiratory effects and route of delivery

Naturally, research on respiratory effects of marijuana does not apply where marijuana is not smoked. Currently there are several alternative methods of administration available including devices with filters, vaporizers, and oral, sublingual, rectal, and transdermal ingestion.

Smoking devices that use water filters (bongs, e.g.) have been shown to involve equivalent amounts of tar and do not reduce risks of marijuana smoke inhalation (Gieringer, 1996; Bloor et al, 2008).  

Vaporizers, which heat marijuana below combustion point, have been theorized to be a safer method of administration, producing lower levels of tar than cigarettes (Grotenhermen, 2001) and fewer respiratory symptoms reported by users (Earleywine & Smucker Barnwell, 2007). However, these devices have also been shown to release ammonia which, when inhaled, can cause irritation and central nervous system effects, as well as asthma and bronchial spasms (Bloor et al, 2008). More research on the potential use of vaporizers as a harm reduction technique is needed.

Eating marijuana (“edibles”) is perhaps a more obvious means to reduce the respiratory effects when using the drug. Oral administration carries its own challenges, however, as it typically takes longer for the effects of the drug to appear (30–60 minutes compared to seconds), making it more difficult to monitor dose and increasing the risk of overdose. Additionally, the effects last longer than some users prefer (Grotenhermen, 2001). That said, overdosing on marijuana is rare and most likely to happen to naïve users. A marijuana overdose can trigger acute anxiety or panic, increased heart rate, low blood pressure, and additional problems.

Other alternative forms of marijuana delivery, including sublingual, rectal, and transdermal delivery have not been appropriately investigated but may also further reduce the possible risks associated with the administration of cannabis (Grotenhermen, 2001).


  • Based on the current scientific research, there is no definitively proven “safe” way of inhaling marijuana (smoke or vapor).
  • Inhaling marijuana smoke in the long-term is likely to result in damage to the respiratory tract.
  • Smoking both marijuana and tobacco increases the risk of damage from both substances.
  • While marijuana smoke contains carcinogens, research about the association between marijuana smoking and cancer is limited and conflicting. The same is true for other lung-related diseases.
  • Marijuana smoking does appear to increase cough, sputum production, airway inflammation, and wheeze.
  • Using a water pipe or bong does nothing to reduce exposure to tar and carcinogens in marijuana smoke; using a vaporizer may reduce the harm associated with smoking marijuana, however vaporizers may carry their own risks and more research is needed.
  • Oral administration of marijuana (“edibles”) is one way to eradicate the risk of marijuana smoke on the lungs, but carries with it its own challenges.


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Updated 2013. This information adapted with permission from the National Cannabis Prevention and Information Centre in Australia.