COMPARING CANNABIS WITH TOBACCO-AGAIN

Link Between Cannabis and Mortality Is Still Not Established

A recent editorial in this journal implied that as many as 30,000
deaths in Britain every year might be caused by smoking cannabis.1 The
authors reasoned that since the prevalence of smoking cannabis is
about one quarter that of smoking tobacco the number of deaths
attributable to smoking cannabis might be about one quarter of the
number attributed to tobacco cigarettes (about 120,000). The idea that
the use of cannabis increases mortality is worthy of closer
examination. How do we assess this issue?

Firstly, we need to examine published data regarding use of cannabis
and mortality. These data come from two large studies. The first study
done in a cohort of 45,450 male Swedish conscripts, age 18-20 when
interviewed about the use of cannabis, reported no increase in the 15
year mortality associated with the use of cannabis after social
factors were taken into account.2 The second study was performed in a
cohort of 65,171 men and women age 15-49, who were members of a large
health maintenance organisation in California, United States. They
completed a questionnaire assessing their use of cannabis, and
reported no increase in mortality associated with use of cannabis over
an average of 10 years of follow up, except for AIDS related mortality
in men.3 A detailed examination showed that the mortality link between
cannabis and AIDS was not a causal one. Thus published data do not
support the characterisation of cannabis as a risk factor for mortality.

Secondly, we need to consider the time course of exposure to cannabis
and its potential relation to mortality. No acute lethal overdoses of
cannabis are known,4 in contrast to several of its illegal (for
example, cocaine) and legal (for example, alcohol, aspirin,
acetaminophen) counterparts. Deaths due to chronic diseases resulting
from substance misuse generally result from the use of that substance
(for example, tobacco and alcohol) over a long time. Importantly, and
in contrast to users of tobacco and alcohol, most cannabis users
generally quit using cannabis relatively early in their adult lives.
The table shows observations from the 1998 US national household
survey on drug abuse regarding the prevalence of current (past month)
use of alcohol, tobacco cigarettes, and use of cannabis among young
adults (age 18-25) and older adults (age 35 or older).5

The proportion of older adults who use cannabis is only 18% that of
younger adults, much lower than the comparable proportions for alcohol
(89%) and tobacco cigarettes (60%). Moreover since the use of cannabis
in young adults declined steadily between 1979 and 1998, whereas use
in older adults remained stable, the observed low prevalence in older
adults is unlikely to increase in the foreseeable future. Therefore,
even diseases that might be related to long term use of cannabis are
unlikely to have a sizeable public health impact because most people
who try cannabis do not become long term users. This observation is
relevant to lung cancer, which, although strongly related to cigarette
smoking, typically only occurs after at least 20 years of smoking.6
Also, a typical regular cannabis user smokes the equivalent of one
marijuana cigarette or less per day,7 whereas consumption of 20 or
more tobacco cigarettes is common. Exposure to smoke is therefore
generally much lower in cannabis than in tobacco cigarette smokers,
even taking into account the larger exposure per puff.8

A third issue to consider is the potential relation of the use of
cannabis to diseases that contribute the most to total mortality. For
example, in the United States and the United Kingdom the leading cause
of death is diseases of the heart, predominantly coronary heart
disease, which is strongly associated with smoking tobacco cigarettes
and accounts for nearly one third of all deaths. Mittleman et al noted
the quadrupling of risk found in one study when cannabis was smoked
within one hour before a myocardial infarction.9 However, since only
0.2% of the patients with myocardial infarction reported this exposure
the number of myocardial infarctions attributable to the use of
cannabis is extremely small. Cannabis does not contain nicotine, a
component of tobacco that contributes importantly to the risk of
coronary heart disease. Use of cannabis in a young adult population
was not associated with the presence of calcium in coronary
arteries--an indicator of coronary atherosclerosis10--and a cohort
study conducted in a large health maintenance organisation showed no
association between the use of cannabis and admission to hospital for
myocardial infarction and all coronary heart disease.11

Two caveats must be noted regarding available data. Firstly, the
longer term follow up of cohorts of cannabis users may still show an
increased risk of cancers, chronic diseases, and mortality if enough
members of the study cohort continue to smoke cannabis often enough
and for long enough. The cohorts to date have not followed cannabis
smokers into later adult life so that it might be too early to detect
an increased risk of chronic diseases that are potentially associated
with the use of cannabis. Secondly, the low rate of regular use of
cannabis and the high rates of discontinuation during young adulthood
in the United States may reflect the illegality and social disapproval
of the use of cannabis. This means that we cannot assume that smoking
cannabis would continue to have the same small impact on mortality (as
it probably does with current patterns of use) if its use were to be
decriminalised or legalised.

Although the use of cannabis is not harmless, the current knowledge
base does not support the assertion that it has any notable adverse
public health impact in relation to mortality. Common sense should
dictate a variety of measures to minimise adverse effects of cannabis.
These include discouraging the use by teenagers, who seem to be most
at risk of future problems from drug use,12 not using before or during
the operation of automobiles or machinery, not using excessively, and
cautioning in people with known coronary heart disease.

REFERENCES

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2. Andreasson S, Allebeck P. Cannabis and mortality among young men: a
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3. Sidney S, Beck JE, Tekawa IS, Quesenberry CP, Friedman GD.
Marijuana use and mortality. Am J Public Health 1997;87: 585-90.

4. Hall W, Solowij N, Lemon J. The health and psychological consequences of
cannabis use. Canberra: National Drug and Alcohol Research Centre, National
Task Force on Cannabis, Australian Government Publishing Service, 1994: 42.
(Monograph series no. 25.)

5. United States Department of Health and Human Services, Office of
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7. United States Department of Health and Human Services, Office of
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10. Sidney S, Kiefe C, Hilner S, Hulley S. Association of lifetime
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12. Robins LN, Przybeck TR. Age of onset of drug use as a factor in drug
and other disorders. In: Jones CL, Battjes RJ, eds. Etiology of drug abuse:
implications for prevention. Rockville, MD: National Institute on Drug
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