Action
on Smoking and Health - The Smoking Gun
November 2002
There
has been extensive coverage of the British Lung Foundation's report
"A smoking gun?"
released to the press on 11 November 2002 in which comparisons are
made between the hazardousness of tobacco and cannabis use. Some
of the media coverage goes beyond what is stated in the report,
but in other areas the report itself is quite misleading. The report
is an in-house publication and is available at the BLF
web site. The report is a literature review, and as such contains
no new data – though it has been reported in some places as
if it is new research. Here are some comments from ASH on the main
headlines...
1.
Murky origin of the claim that three cannabis joints equates to
20 cigarettes
The
reported claim that three cannabis joints per day equate in risk to
20 cigarettes has a very dubious basis. It is based on data published
in two papers in 1987 (see below), but the papers in question did
not make that claim and were MUCH more cautious. This relationship
was asserted in a later paper that cited the original papers as demonstrating
the relationship without adding any new analysis, even though the
originals did not contain that interpretation. Note that although
the original data papers were actually cited in the BLF report in
relation to other points, the BLF did not cite these papers in support
of the 3:20 claim – one might assume because these papers didn't
make the claim. The evolution of the 3:20 assertion is set out below,
starting from the news coverage on launch day, and working back through
press release, the report, and its references to secondary sources
to find its original source.
2.
Limited scope of comparison between tobacco and cannabis
The
two 1987 studies on which the claim is based examine only a limited
range of respiratory illness symptoms, and did not estimate the
risks of lung cancer and chronic obstructive pulmonary disease (COPD
- eg. emphysema), which are the main fatal lung diseases caused
by smoking tobacco. In the UK, lung cancer and COPD are responsible
for almost half of tobacco related deaths (heart disease taking
most of the rest). The BLF report acknowledges "conflicting
findings" on the link between lung cancer and cannabis, and
calls for more research "to establish what link (if any) there
is between COPD and cannabis smoking". Given that the data
used don't actually cover the main risks and the link between these
major risks is acknowledged to be uncertain, it is premature to
draw overall risk comparisons between cannabis and tobacco - and
certainly not with precision like 3:20. BLF did limit the scope
of the claim in its report - but it was widely interpreted as a
measure of overall relative risk.
However,
one can see how this happens... for example: on BBC Radio's flagship
"Today" programme (11 November 2002, 08.28):
Presenter:
"...you come to the conclusion that three cannabis joints a
day are doing the same damage as 20 cigarettes?"
BLF spokesperson: "Absolutely".
3.
Central importance of usage pattern and lifetime exposure ignored
The
major mortal risks to the lungs, heart and circulation depend on
lifetime exposure as well as the toxicity of the smoke. The importance
of lifetime exposure was underlined in a major study of tobacco
smokers in 2000 which found that: "People who stop smoking,
even well into middle age, avoid most of their subsequent risk of
lung cancer, and stopping before middle age avoids more than 90%
of the risk attributable to tobacco." [*Peto R et al. Smoking,
smoking cessation, and lung cancer in the UK since 1950: combination
of national statistics with two case-control studies. BMJ 2000;
321: 323-329]. The very high risks due to tobacco use really arise
from its addictiveness, which causes many tobacco smokers to continue
to smoke well after they would want to stop. It is common for tobacco
smokers to consume 20 cigarettes per day every day for several decades.
However, this is not a common pattern of use for cannabis, which
appears to be much less addicting than nicotine. The UK government’s
Advisory Council on Misuse of Drug noted that:
In
general cannabis users smoke fewer cigarettes per day than tobacco
smokers and most give up in their 30s, so limiting the long-term
exposure that we now know is the critical factor in cigarette-induced
lung cancer. (4.4.1)
Advisory
Council on the Misuse of Drugs. The
classification of cannabis under the Misuse of Drugs Act 1971,
(UK Government) Home Office, March 2002.
The
House of Lords took evidence on addictiveness of cannabis and concluded
that dependence was less serious:
Giving
up cannabis is widely believed to be relatively easy: according
to the Department of Health, "studies report that of those
who had ever been daily users only 15 per cent persisted with daily
use in their late twenties" (4.31)
House
of Lords Committee on Science and Technology, Ninth Report Session
1997-98, Cannabis,
the scientific and medical evidence. November 1998.
4.
Unwarranted and unsubstantiated scare about increased strength of
modern cannabis
The
report claims that: "The cannabis smoked today is much more
potent than that smoked in the 1960s" and states that the average
cannabis cigarette in 1960s contained 10mg THC (the active substance)
compared to 150mg today. However, this increase in the strength
of cannabis is not obviously a cause for alarm. Even if there is,
as claimed, 15 times as much THC in modern cannabis it is unlikely
that today's users are 15 times as stoned as their predecessors
in the 1960s. It is plausible that cannabis users control the dose
they receive by varying their smoking pattern - as it has been shown
that tobacco smokers do for nicotine. Stronger cannabis may therefore
mean that LESS smoke is inhaled for a given dose of the active ingredient.
There is a large literature on 'compensation' and the tendency of
smokers to titrate nicotine, though the subject is much less well
understood for cannabis. A better working assumption that dose is
controlled by the user rather than by the cigarette, and anecdotally,
cannabis smokers say that they smoke rather than eat the drug because
it is easier to control the dose. However, the report jumps without
evidence to the opposite conclusion, leading to some extremely misleading
reporting – for example on CNN, one of the world’s largest
news networks made the following report, which is thoroughly flawed.
See Cannabis
smoke health warning, 11 November 2002.
LONDON,
England --Health risks from smoking cannabis have risen dramatically
since the 1960s because of changes to the way the drug is produced,
a health charity says. […] The BLF report, published on Monday,
said the health risks were worse now than in the 1960s because there
is more THC (tetrahydrocanabinol), the ingredient which accounts
for the psychoactive properties of cannabis, in the substance consumed
today.
5.
Claimed higher toxicity of cannabis tar ignores dramatic variations
in tobacco toxin concentrations
The
claim that there are 50% more carcinogens in cannabis tar in tobacco
smoke also demands caution given the wide variation in carcinogens
within even the same cigarette brand. The BLF report identified
two carcinogens - "tar from cannabis cigarettes contains up
to 50% higher concentrations of the carcinogens benzanthracenes
and benzpyrenes". But carcinogens do vary wildly within cigarette
brands... Gray et al (2000) measured two nitrosamine yields, NNK
and NNN, in Camel, Marlboro and Lucky Strike cigarettes, and found
that a:
"three
to nine fold variation in carcinogen dose can be given to the smoker...
".
Gray
N, Zaridze D, Robertson C, et al. Variation
within global cigarette brands in tar, nicotine, and certain nitrosamines:
analytic study. Tob Control 2000;9: 351.
Measurements
made in British Columbia also show marked carcinogen variations
between brands of similar magnitude to that reported for cannabis
- see: British
Columbia Tobacco Testing and Disclosure: What's in Cigarettes?
6.
Need for education…
The
BLF report states that “The British Lung Foundation recommends
a public health education campaign aimed at young people…”.
However, misleading and simplistic comparisons of risk, and wholly
flawed interpretation of data related to ‘strength’ of
cannabis do little to educate or inform anyone – young or old.
Most of the criticisms above were drawn to the attention of BLF,
when a preview of the “A smoking gun?” was reported in
the Mail on Sunday on 21 July 2002. ASH provided comments to BLF
at the time making most of the points above on the basis of the
news article and urged caution about drawing these conclusions.
These comments were ignored. In July 2002 The British Lung Foundation
was even more explicitly claiming that cannabis was more harmful
than tobacco. See BBC report of 10 July 2002: Cannabis
“worse than tobacco”
BLF
chief executive Dame Helena Shovelton said: "Many young people
are simply not aware that smoking cannabis may put them at increased
risk of respiratory cancers and infections. The government spends
millions of pounds a year on smoking cessation and public education
about the dangers of smoking, yet smoking cannabis is at least as
harmful as smoking tobacco and, indeed, may carry a higher risk
of some respiratory cancers."
Despite
this message, delivered earlier in the year in response to the government’s
plans to decriminalize cannabis, the report as published does not
substantiate that claim. It is difficult therefore to see much educative
value in these pronouncements, especially as they contrast with
more carefully and credibly formulated information and analysis
in recent independent assessments, some of which we describe below.
The
usually cautious Advisory Council on Misuse of Drugs, which concluded
in March 2002 after a thorough review of the evidence...
"The
high use of cannabis is not associated with major health problems
for the individual or society." (5.1)
Advisory
Council on the Misuse of Drugs. The
classification of cannabis under the Misuse of Drugs Act 1971,
(UK Government) Home Office, March 2002.
Further,
in 1998 The Lancet commissioned a thorough review of the health
impacts of cannabis use and concluded in an editorial:
…on
the medical evidence available, moderate indulgence in cannabis
has little ill-effect on health, and that decisions to ban or to
legalise cannabis should be based on other considerations.
Anon
(editorial). Dangerous
habits. Lancet 1998;352:1565.
Hall
W. and Solowij N. Adverse
effects of cannabis, Lancet 1998; 352:1611-16
The
Police Foundation’s extensive inquiry (‘The Runciman Commission’)
concludes:
When
cannabis is systematically compared with other drugs against the
main criteria of harm (mortality, morbidity, toxicity, addictiveness
and relationship with crime), it is less harmful to the individual
and society than any of the other major illicit drugs or than alcohol
and tobacco.
Drugs
and the Law, Report of the Independent Inquiry into the Misuse of
Dugs Act (1971): Chairman: Viscountess Runciman DBE, 1999.
For
a discussion of wider issues in relation to cannabis and tobacco,
analysis of policy implications and extensive links to reviews of
scientific evidence see ASH’s: Legalisation of cannabis –
a discussion document.
Warning!
– and further information
None of this is to argue that cannabis is harmless! Far from it.
Prolonged and heavy cannabis use should be expected to cause respiratory
diseases and other ill-effects. However, when making a comparison
with tobacco or educating tobacco and cannabis users about the risks
it is important to have the harm in perspective. For a better understanding
of the risks associated with cannabis we advise consulting the sources
listed above and a special reviews in:
Journal
of Clinical Pharmacology 42 (11 supplement), November 2002 (abstracts)
Advisory Council on the Misuse of Drugs. The
classification of cannabis under the Misuse of Drugs Act 1971,
(UK Government) Home Office, March 2002.
Hall W. and Solowij N. Adverse
effects of cannabis, Lancet 1998; 352:1611-16
For
an understanding of tobacco, we advise consulting the following:
UK
Scientific Committee on Tobacco and Health
US
Surgeon General reports
Royal
College of Physicians: Nicotine Addiction in Britain
Appendix
Finding the source of the claim that three cannabis cigarettes
are as dangerous as 20 tobacco cigarettes
This
is the evolution of the 3:20 claim, tracing it back to the data
papers published in 1987.
1.
The reporting...
"The
evidence indicates that three cannabis joints does the same damage
to the lining of the lungs as 20 cigarettes" The Independent
"Smoking three joints per day might do as much damage to the
lungs as do 20 cigarettes" The Guardian
"Three or four cigarettes are as damaging as 20 cigarettes"
The Times
"Three or four cannabis cigarettes are equivalent to smoking
20 tobacco cigarettes a day in terms of the risk of lung damage"
The Telegraph
"A study by the British Lung Foundation found that just three
cannabis joints a day cause the same damage as 20 cigarettes"
BBC.
2.
BLF Press release
"Three
cannabis joints a day cause the same damage to the lining of the
airways as 20 cigarettes."
3.
BLF Report Summary
"3-4
Cannabis cigarettes a day are associated with the same evidence
of acute and chronic bronchitis and the same degree of damage to
the bronchial mucosa as 20 or more tobacco cigarettes a day.
4.
BLF report body
"It
has been calculated that smoking 3-4 cannabis cigarettes a day is
associated with the same evidence of acute and chronic bronchitis
and the same degree of damage to the bronchial mucosa as 20 or more
tobacco cigarettes a day [14][ 15].
5.
Reference 15 used in BLF report
Ref
15 in BLF report is: Tashkin, DP, Effects of marijuana smoking profile
on respiratory deposition of tar and absorption of CO and D-9 tratrahydrocanabinol,
In: Pulmonary pathophysiology and immune consequences of smoked
substance abuse, FASEB Summer Research Conference, July 18-23, 1999,
Copper Mountain, CO
This
not a peer reviewed paper and not easy to obtain. However, Tashkin
is an author of the papers that offer the original data.
6.
Reference 14 used in BLF report
Ref
14 in BLF report: Ashton H, 2001 Pharmacology and effects of cannabis:
a brief review Br Journal of Psyschiatry 178, 101-106
Ashton
doesn’t make the case herself, but cites an earlier paper,
(Benson & Bentley, 1995) to make this claim.
"It
has been calculated that smoking 2-4 cannabis cigarettes a day is
associated with the same evidence of acute and chronic bronchitis
and the same degree of damage to the bronchial mucosa as 20 or more
cigarettes a day (Benson and Bentley, 1995)
7.
Benson & Bentley (cited as a source in BLF ref 14)
Benson
M and Bentley AM, (1995). Lung disease induced by drug addiction.
Thorax, 50, 1125-1127 - cited in Ashton (2001)
Benson
& Bentley's article does not contain this calculation or make
this claim. The nearest Benson and Bentley get is to cite the ubiquitous
Wu et al (1988) study...
All
Benson and Bentley say is "Smoking a cannabis cigarette results
in an approximately five-fold greater increase in carboxyhaemoglobin
concentration than with a tobacco cigarette, with increases in inhaled
tar content and the amount retained in the respiratory tract. [23]".
This is a repeat of the Wu et al conclusion and does not in itself
substantiate the 3:20 relationship.
8.
Wu et al (1998) cited in Benson and Bentley as ref 23
Wu
TC, Tashkin DP, Djahed B, et al. Pulmonary hazards of smoking marijuana
as compared with tobacco. N Engl J Med 1988;318: 347-351.
The
Wu et al paper is also cited in the BLF report at ref 16 - but this
is not used to substantiate the 3:20 estimate. Wu et al. measured
tar deposition in the lungs of 15 smokers of both cannabis and tobacco.
The Wu et al paper is also not the original source, but refers to
an earlier claim and starts with the following...
"We
have previously shown that the habitual smoking of 3 or 4 marijuana
cigarettes per day is associated with the same frequency of the
symptoms of acute and chronic bronchitis [1] and same type and extent
of epithelial damage in the central airways [2] as the regular smoking
of 20 tobacco cigarettes a day.
9.
The original data papers
It
seems that references [1] and [2] in Wu et al are the original data
sources for this claim...
[1]
Tashkin, DP, Coulson, AH, Clark, VA, et al, 1987, 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 135, 209-216
[2]
Gong, H, Fligiel, S, Tashkin, DP, Barbers, RG, 1987, Tracheobronchial
changes in habitual heavy smokers of marijuana with and without
tobacco, Am Rev Respir Dis 136, 209-216
The
abstracts of these papers are reproduced below. Neither paper
makes a numerical comparison between cannabis and tobacco.
These
papers are referenced at [38] and [33] respectively in the BLF report,
but are not used in the BLF report as citations to back the 3:20
claim. One good reason for that may be that these papers do not
actually make this claim. It is not until these papers are cited
in the introduction to Wu et al, that the 3:20 (or more accurately
3 or 4 to 20) comparison starts to be made, and this is then subsequently
repeated in other later papers as if it is established. The author
that is common to the two data papers, and to Wu et al is Donald
Tashkin - and Tashkin's conference paper is also cited by BLF in
support of this comparison. Why the claim was not included in the
basic papers but then cited as fact subsequently is unclear to me.
Possible
reason why the 3:20 interpretation was not made in the original
paper
A possible reason why the claim wasn't made (or accepted by peer
reviewers) in the original data paper is as follows: There are only
really two data points - these are the average consumption of the
tobacco smokers and average consumption of the marijuana smokers
recruited into the study (3.9 and 22 per day respectively in [1]).
There was no attempt in either paper to see how the symptoms varied
with consumption. So it is unknown whether people smoking three
tobacco cigs per day would have experienced the same or less of
the symptoms than someone smoking 20 cigs. If the symptoms were
not that sensitive or proportional to consumption, then entirely
different number may have been found. These are also unusually heavy
users of cannabis but quite 'normal' users of tobacco and it is
possible can imagine quite a bit of potential confounding and potential
bias.
Abstracts
for the original papers
It may be of interest to know what was actually said in the data
papers: it turns out to be far more cautious.
Abstract
for Tashkin et al [1] above...
To
evaluate the possible pulmonary effects of habitual marijuana smoking
with and without tobacco, we administered a detailed respiratory
and drug use questionnaire and/or lung function tests to young,
habitual, heavy smokers of marijuana alone (n = 144) or with tobacco
(n = 135) and control subjects of similar age who smoked tobacco
alone (n = 70) or were nonsmokers (n = 97). Mean amounts of marijuana
and/or tobacco smoked were 49 to 57 joint-years marijuana (average
daily number of joints times number of years smoked) and 16 to 22
pack-years of tobacco. Among the smokers of marijuana and/or tobacco,
prevalence of chronic cough (18 to 24%), sputum production (20 to
26%), wheeze (25 to 37%) and greater than 1 prolonged acute bronchitic
episode during the previous 3 yr (10 to 14%) was significantly higher
than in the nonsmokers (p less than 0.05, chi square). No difference
in prevalence of chronic cough, sputum production, or wheeze was
noted between the marijuana and tobacco smokers, nor were there
additive effects of marijuana and tobacco on symptom prevalence.
We noted significant worsening effects of marijuana but not to tobacco
on specific airway conductance and airway resistance (tests of mainly
large airways function) in men and of tobacco but not of marijuana
on carbon monoxide diffusing capacity and on closing volume, closing
capacity, and the slope of Phase III of the single- breath nitrogen
washout curve (tests reflecting mainly small airways function) (p
less than 0.03, two-way ANCOVA). No adverse interactive effects
of marijuana and tobacco on lung function were found
Abstract
for Gong et al [2] above...
We
performed flexible fiberoptic bronchoscopy in 29 habitual, heavy
marijuana smokers 25 to 45 yr of age, with and without concomitant
tobacco smoking, to inspect and biopsy their proximal tracheobronchial
tree for the evaluation of histopathologic changes. Control tobacco
smokers (TS) and nonsmokers (NS) residing in the same metropolitan
area were similarly studied and compared with the marijuana smokers
(MS) and marijuana-tobacco smokers (MTS). Respiratory and drug histories,
physical examination, and pulmonary function tests were obtained
prior to bronchoscopy. The prevalence of respiratory symptoms and
pulmonary function abnormalities was generally higher in the 3 smoking
groups than in the NS group but was not statistically different
across all groups. However, bronchoscopic inspection revealed airway
hyperemia and other visible abnormalities in 32 (91%) subjects in
the 3 smoking groups, unlike the unremarkable findings in the NS
group. Light microscopy showed 2 or more histopathologic changes
in the bronchial epithelium of all MS, MTS, and TS. Squamous metaplasia
was observed in all MTS, a prevalence that was significantly different
from that in MS, TS, and NS. Hyperplasia of basal and goblet cells
was more prevalent in the MS than in the NS, whereas cellular disorganization
was more prevalent in the MS than in the TS. A direct relationship
between cumulative marijuana use (joint-years) and bronchoscopic
and histopathologic changes was not apparent in this study sample.
These results indicate that relatively young, habitual, heavy marijuana
smokers have a high prevalence of abnormal airway appearance and
histologic findings, irrespective of concomitant tobacco smoking.
the results suggest a causal relationship between marijuana smoking
and histologic lesions in the airways. The long term clinical importance
of these histopathologic findings is unclear. In view of the widespread
use of marijuana in this country, however, the finding of histopathologic
airway change in young adult marijuana smokers justifies a serious
concern about the development of chronic airy disease in these smokers.
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