Select Committee on Home Affairs Memoranda


Submitted by the Royal College of Physicians

Report prepared by Professor John Britton, Division of Respiratory Medicine, City Hospital, Nottingham, on the long-term adverse effects of smoking marijuana on lung health


  Marijuana smoking exposes the lung to toxic products of combustion more commonly associated with tobacco smoke, and is therefore likely to be associated with similar long-term health risks as tobacco smoking. The potential harm reduction of the lower frequency of smoking marijuana compared with tobacco is likely to be appreciably offset by the greater degree of inhalation and deposition of marijuana combustion products.

  The overall evidence available to date suggests that marijuana smokers are likely to be at increased risk of the same major lung diseases as tobacco smokers. Epidemiological studies confirm that they are at greater risk of chronic bronchitis symptoms and airflow obstruction. No objective evidence of increased risk has yet been presented for lung cancer, but this and the absence of evidence for an effect on total mortality may have more to do with the relatively narrow evidence base and the short duration of follow-up available in studies to date than a true absence of risk.

  The similarity of the effects of marijuana and tobacco smoke on the airway make it highly likely that lung cancer will prove, in time, to be a major adverse effect on marijuana smoking, and the same is likely in relation to cardiovascular disease. Widespread use of smoked marijuana is therefore likely to result in significant adverse effects on public health.


  Whilst the adverse adverse health effects of smoking tobacco are well recognised1 the adverse effects of marijuana smoking are generally less well understood. However there is increasing evidence that regular marijuana smoking does have, or will have, significant and clinically important effects on the risk of chronic obstructive pulmonary disease, lung cancer, head and neck cancer, and possibly other disease. These effects have been the subject of a recently and authoritative review by one of the major international experts on this topic2. This outline report draws heavily on the findings of that review, and cites the major references supporting those finding.


  Marijuana is probably most commonly smoked in hand-rolled joints in which marijuana resin or leaf is combined with tobacco. Marijuana is also smoked in pure form in joints, pipes or other devices3. Compared to tobacco smoking the following differences in smoking pattern, and consequences or relevance to health, apply to marijuana:

    —  The number of joints smoked per day is usually substantially less than the number of cigarettes smoked by a pure tobacco smoker. The total amount of smoke inhaled is therefore less with marijuana.

    —  The absence of filters in the smoking devices means that a higher proportion of tar (the constituents of which are very similar to tobacco tar, except that marijuana smoke does not contain nicotine) is inhaled.

    —  Smoke from joints or other sources tends to be inhaled more deeply, and held in the lungs for longer, than pure tobacco smoke, so deposition of combustion products in the lung is proportionately greater4.

    —  Some marijuana smokers attempt to increase drug absorption by performing a valsalva manoeuvre after deep inhalation, which may lead to local barotrauma in the lung5.

  As a result, the potentially lower health hazard (relative to tobacco smoking) of a lower frequency of marijuana smoking is offset to a substantial degree by differences in inhalation practice and the lack of filtration. Marijuana smoke is also hotter than tobacco smoke, which is also more damaging.


  Given that most marijuana is smoked in a mixture with tobacco, most marijuana smokers are also exposed to:

    —  All of the recognised health risks of smoking tobacco resulting from the tobacco smoked with the marijuana.

    —  The risk of establishing or established nicotine addiction as a result of smoking joints containing tobacco as well as marijuana, and the consequent adverse health effects of the tobacco smoking habit that results.

  The fact that marijuana is so often smoked with tobacco, and that marijuana smokers are often also regular cigarette smokers6, means that the independent effects of these different exposures can be difficult to distinguish. The evidence cited below in relation to marijuana effects is taken from studies in which attempts have been made to distinguish the independent effect of marijuana smoking either by statistical adjustment, or by restriction to populations of marijuana smokers who do not smoke tobacco.


  Marijuana smokers show evidence of a higher frequency of epithelial, basement membrane and submucosal abnormalities than non-smokers7 and a greater frequency of mucosal and basement membrane abnormalities8, and cellular disorganisation9 than tobacco smokers. This and other evidence of increased airway inflammation in marijuana smokers10 provides evidence that marijuana smoking is likely to be associated with an increased risk of lung cancer and chronic obstructive pulmonary disease.


  Marijuana smokers have an increased risk of chronic cough, production of sputum, shortness of breath and wheeze, of a magnitude of the order of 1.5 to 2-fold relative to non-smokers11-14.


  In cross-sectional studies, marijuana smoking is associated with reduced levels of lung function consistent with airflow obstruction12-14. However only one study has found evidence that this translated into an accelerated longitudinal decline in lung function in marijuana smokers13. One other study found no evidence of accelerated decline, though this study was carried out in a relatively small number of subjects and may have been underpowered to detect a relevant effect15. The occurrence of large lung bullae (findings that probably share a similar pathophysiology to emphysema) has also been reported in marijuana smokers5, which may result from different inhalation practices, but could also be a manifestation of an increased risk of emphysema in marijuana smokers.


  Marijuana smoke contains several of the carcinogens present in tobacco smoke, including vinyl chlorides, phenols, nitrosamines, reactive oxygen species and polycyclic aromatic hydrocarbons (summarised by Tashkin2), and deposition of these carcinogens in the lung is higher than for tobacco smoke for the reasons given at (2) above. There is extensive evidence that marijuana smoke causes pathological changes at a cellular level consistent with carcinogenesis in the human airway2, though to date there is no reported evidence of an increased risk of lung cancer in man. However marijuana smokers have been reported to be at increased risk of head and neck cancer16, and also prostate and cervical cancer17. It therefore seems likely that in time, lung cancer will also emerge as a significant problem in marijuana smokers.


  The effects of marijuana smoking on cardiovascular disease are less clearly established. However the fact that marijuana smoking generates much higher levels of carbon monoxide than tobacco smoke4, 18, and has effects on HDL-cholesterol, triglycerides and phospholipids that are also likely to increase the risk of cardiovascular disease19, makes it likely that significant effects on cardiovascular disease will become apparent.


  The single study to address total mortality in marijuana smokers data has found no evidence of increased mortality, other than from HIV-related disease, in habitual marijuana smokers20.

September 2001


  1.  Royal College of Physicians. Nicotine Addiction in Britain. A report of the Tobacco Advisory Group of the Royal College of Physicians. London: Royal College of Physicians of London, 2000.

  2.  Tashkin DP. Airway effects of marijuana, cocaine, and other inhaled illicit agents. Curr.Opin.Pulm.Med 2001;7:43-61.

  3.  Ashton CH. Pharmacology and effects of cannabis: a brief review. Br.J Psychiatry 2001;178:101-6.

  4.  Wu TC, Tashkin DP, Djahed B, Rose JE. Pulmonary hazards of smoking marijuana as compared with tobacco. N.Engl.J Med 1988;318:347-51.

  5.  Johnson MK, Smith RP, Morrison D, Laszlo G, White RJ. Large lung bullae in marijuana smokers. Thorax 2000;55:340-2.

  6.  Simmons MS,. Tashkin DP. The relationship of tobacco and marijuana smoking characteristics. Life Sci 1995;56:2185-91.

  7.  Fligiel SE, Roth MD, Kleerup EC, Barsky SH, Simmons MS, Tashkin DP. Tracheobronchial histopathology in habitual smokers of cocaine, marijuana, and/or tobacco. Chest 1997;112:319-26.

  8.  Fligiel SE, Venkat H, Gong H. Jr., Tashkin DP. Bronchial pathology in chronic marijuana smokers: a light and electron microscopic study. J Psychoactive Drugs 1988;20:33-42.

  9.  Gong H, Jr., Fligiel S, Tashkin DP, Barbers RG. Tracheobronchial changes in habitual, heavy smokers of marijuana with and without tobacco. Am Rev Respir Dis 1987;136:142-9.

  10.  Roth MD, Arora A, Barsky SH, Kleerup EC, Simmons M, Tashkin DP. Airway inflammation in young marijuana and tobacco smokers. Am J Respir Crit Care Med 1998;157:928-37.

  11.  Tashkin DP, Coulson AH, Clark VA, Simmons M, Bourque LB, Duann S et al. Respiratory symptoms and lung function in habitual heavy smokers of marijuana alone, smokers of marijuana and tobacco, smokers of tobacco alone, and nonsmokers. Am Rev Respir Dis 1987;135:209-16.

  12.  Bloom JW, Kaltenborn WT, Paoletti Peal. Respiratory effects of non-tobacco cigarettes. Br Med J 1987.

  13.  Sherrill DL, Krzyzanowski M, Bloom JW, Lebowitz MD. Respiratory effects of non-tobacco cigarettes: a longitudinal study in general population. Int J Epidemiol 1991;20:132-7.

  14.  Taylor DR, Poulton R, Moffitt TE, Ramankutty P, Sears MR. The respiratory effects of cannabis dependence in young adults. Addiction 2000;95:1669-77.

  15.  Tashkin DP, Simmons MS, Sherrill DL, Coulson AH. Heavy habitual marijuana smoking does not cause an accelerated decline in FEV1 with age. Am J Respir Crit Care Med 1997;155:141-8.

  16.  Zhang ZF, Morgenstern H, Spitz MR, Tashkin DP, Yu GP, Marshall JR et al. Marijuana use and increased risk of squamous cell carcinoma of the head and neck. Cancer Epidemiol Biomarkers Prev. 1999;8:1071-8:

  17.  Sidney S, Quesenberry CP, Jr., Friedman GD, Tekawa IS. Marijuana use and cancer incidence (California, United States). Cancer Causes Control 1997;8:722-8.

  18.  Tashkin DP, Wu TC, Djahed B. Acute and chronic effects of marijuana smoking compared with tobacco smoking on blood carboxyhemoglobin levels. J Psychoactive Drugs 1988;20:27-31.

  19.  Kalofoutis A, Papapanagiotou A, Hatjioannou A, Maravelias K. Changes in human serum high density lipoprotein induced by hashish constitutents. Medical Science Research 1999;27:117-8.

  20.  Sidney S, Beck JE, Tekawa IS, Quesenberry CP, Friedman GD. Marijuana use and mortality. Am J Public Health 1997;87:585-90.

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Prepared 20 December 2001