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
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
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
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
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.
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