THE THC CONTENT OF CANNABIS IN AUSTRALIA: EVIDENCE AND IMPLICATIONS

A number of commentators in the alcohol and other drugs field have recently
claimed that the THC (tetrahydrocannabinol) content of cannabis products
used in Australia has increased up to 30 times over the past two decades.
[1-3] The increased THC content has been attributed to the cultivation of
particular cannabis strains, including hybrids of the Cannabis sativa plant
such as 'skunk', and the use of hydroponic growing methods.

Since THC is the psychoactive substance in cannabis that is responsible for
most of its effects, [4] these claims have been used to explain an apparent
increase in the adverse health and psychological effects of cannabis use
among young people who are regular users of the drug.

These claims have been widely and uncritically reported in the popular
media and have played a prominent role in recent debates about proposed
changes to legal penalties for cannabis use. They accordingly require
critical analysis. In this report, we examine evidence on three linked claims:

1. That the THC content of Australian cannabis plants has increased up to
30 times.

2. That problems related to cannabis use have increased among young adults
in Australia in recent years.

3. That increased THC content is the most plausible explanation of any rise
in the rate of cannabis-related problems.

Method

To address the first question, we investigated the availability of
published data on the THC content of cannabis collected by government
analytical laboratories, police and health departments in New South Wales
(NSW), Queensland, South Australia (SA), Victoria and Western Australia
(WA). We also examined data reported in the National Illicit Drug Reports
compiled by the Australian Bureau of Criminal Intelligence. We also
obtained data on the THC content of cannabis in the United States (US) and
New Zealand (NZ).

The second question was addressed by examining available morbidity data on:
the number of hospitalisations for cannabis-- related diagnoses (e.g. abuse
and dependence); rates of addiction treatment for cannabis-related problems
reported in national censuses of treatment agencies; and surveys of the
prevalence of cannabis use among high-risk populations, such as young
people in juvenile justice settings, young people with first episode
psychoses, and young people who are being treated for alcohol and other
drug problems.

We explored alternative explanations for any increase, real or apparent, in
cannabis-- related problems by undertaking secondary analyses of
self-report data on cannabis use, type of cannabis consumed and method of
use from the 1998 National Drug Strategy (NDS) Household Surveys (a
household survey of 10,030 Australians aged 14 years and older). The unit
record file of the NDS survey was purchased from the Social Science Data
Archive. All data are weighted to the Australian population using weights
provided with the NDS unit record file.

Results

Has The Average THC Content Of Australian Cannabis Plants Increased?

The THC content of Australian cannabis products has not been systematically
tested over the two decades in which average THC content is claimed to have
increased up to 30 times. Penalties under law for cannabis offences in
Australian States do not distinguish between cannabis products of differing
potency, so there is no reason for police services to test THC content.

Samples of seized cannabis are occasionally tested. In NSW, for example,
the Division of Analytical Laboratories analysed a sample of compressed,
hydroponically grown 'heads' from SA, which had a THC content of 15%. [6]
Media publicity about the THC content of single samples such as this can
create a false impression that these levels are typical of the cannabis
products consumed in Australia.

Other Australian data on cannabis samples tested for other purposes
suggests that the above sample had an unusually high THC level. In 1997, a
small number of cannabis seizures (leaf and head) from a number of
Australian States were tested for THC content. [7] Their THC content was
between 0.6-13% of plant material, with the majority of samples having a
THC content of 0.6-2.5%. The mean THC content of 168 samples tested by
Western Australian police between March and May 1996 was 3.8% for all
samples, and 6.4% in 59 samples of 'heads'. [8]

Better data have been collected on the THC potency of cannabis in the US,
where a similar claim of a 30-fold increase in cannabis potency has been
made. [9] The Research Institute of Pharmaceutical Sciences at the
University of Mississippi has tested the THC content of cannabis seizures
for the National Institute on Drug Abuse since the middle 1970s (The
Potency Monitoring Project). [10,11] These data suggest that THC content in
US cannabis increased between 1975 and 1998. Mikuriya and Aldrich, however,
have argued that samples from the middle 1970s were not representative of
cannabis consumed at that time and that the samples from the 1970s
under-estimated THC potency because they were not properly stored, allowing
their THC content to degrade. [12] Data from the Marijuana Potency
Monitoring Project have shown an increase in the THC content of 31,000
samples from 1.2% in 1980 to 4.2% in 1997. [10]

The NZ Government has tested the THC content of cannabis samples over the
past two decades. These have not shown any sizeable increase in average THC
content which has remained within the range of 2.0% to 4%. [13]

Why Is It Believed That The THC Content Of Cannabis Has Increased?

A number of factors probably explain the persistence of the belief that the
THC content of cannabis plants in Australia has increased 30-fold in the
absence of any supporting data.

First, defenders of the claim often point to reports of single samples with
unusually high THC content tested by the police. At best, such samples
indicate the maximum THC content that has been achieved (assuming that
there were no errors in the test results) but they do not tell us what the
THC content is in the cannabis that is typically used by consumers.

Second, biases in the sampling of tested cannabis are amplified by the
attention that the print and electronic media give to unusually potent
samples, creating a false impression that cannabis with exceptionally high
THC is the norm.

Third, uncontested repetition of these assertions in the media has
established them as 'facts'; those who contest these claims are asked to
prove that they are false rather than the (usually nameless) proponents
being asked to provide evidence that they are true.

Fourth, an increase in average THC content seems to explain an apparent
increase in the number of cannabis users who experience problems as a
consequence of their use. [14]

Has The Prevalence Of Cannabis-Related Problems Increased In Australia?

Data from household and school-based surveys show that cannabis use has
substantially increased among young Australians over the past 20 years.
[15-17] If the experience with alcohol is applicable to cannabis, then a
rising prevalence of use would predict increased rates of problems. [18]
There are, however, no indicators of cannabis-related mortality and
morbidity that are as directly related to cannabis use as alcoholic liver
cirrhosis and alcoholic psychosis are to alcohol and overdose deaths are to
heroin use. Cannabis-related problems, such as cannabis-induced psychosis
and cannabis dependence are contested entities that are not well recognised
clinically. [19,20]

A number of indicators suggest that cannabis-related problems have probably
increased. National censuses of Australian addiction treatment services
indicate that the proportion of persons presenting for a primary
cannabis-related problem has steadily increased from 4% in 1990 to 7% in
1995. [21] A recent study of drug and alcohol services for the National
Minimum Data Set Project found that 10.8% of 1395 clients sought help for a
cannabis problem in 1997. [22] The number of first-time hospital admissions
for cannabis abuse and dependence among young non-- Aboriginal adults in
Western Australia substantially increased from 185 in 1980-1985 to 1617 in
1990-1995. [23]

There are also high rates of cannabis use among subgroups of young
Australians. For example, 25% of a nationally representative sample of
persons with psychotic disorders in contact with services had a lifetime
history of cannabis abuse and 24% were daily or near-daily cannabis users
[24] compared with 2% in the general population. [25] High rates of daily
cannabis use are also reported among persons with first episode psychoses,
juvenile offenders before the courts and adolescents in treatment programs
for alcohol and drug problems. [26-28] While we do not have good data on
the prevalence ofthese disorders over time, the high prevalence of heavy
cannabis use in these populations has contributed to a perception that
cannabis-related problems have increased among young Australians.

Alternative Explanations Of The Apparent Increase In Cannabis-Related Problems

If we assume for the purposes of argument that cannabis-- related problems
have increased in prevalence, it does not follow that an increase in the
THC content produced by cannabis plants is the most plausible explanation.
Two more plausible alternative explanations of this apparent increase in
cannabis-related problems are: cannabis users more often use more potent
forms of cannabis that dominate the cannabis market in Australia; and an
earlier age of initiation and heavier patterns of cannabis use among young
Australians have increased the prevalence of harmful patterns of cannabis use.

Changing Cannabis Markets

The THC content of cannabis varies between different cannabis products.
Cannabis leaf contains the least THC and the flowering 'heads' contain the
highest amount of THC. [29] Cannabis resin harvested from the flowering
heads and compressed into hash is one of the most potent forms of cannabis.
[30] Leaf, heads and hash have been available in Australia for several
decades. [31] So too have more potent strains of marijuana. The Mullumbimby
and the Byron Bay districts of northern NSW, for example, had a reputation
for producing high-potency cannabis known as 'Mullumbimby madness' in 1981.
[32]

We need to distinguish between two ways in which THC content may have
increased: more potent THC-producing strains of cannabis plant (e.g.
'skunk'), and more potent cannabis products derived from existing plants.
The popular media give greatest attention to the first possibility; the
data suggest that the latter is more plausible.

Over the past two decades, a large-scale illicit cannabis industry has
developed in Australia [33] to meet the demand for cannabis among a growing
number of regular cannabis users. In the 1998 NDS Survey, persons who
smoked cannabis weekly, or more frequently, comprised 31% of those who had
used cannabis in the past year but they accounted for an estimated 96% of
the cannabis consumed making conservative assumptions about their frequency
of use by daily users.

Regular users generally prefer the more potent forms - heads of the plant
[34,35] - probably because they develop tolerance to the effects of THC.
[29,36] In the 1998 NDS Survey, 94% of daily and 88% of weekly cannabis
users reported that they typically smoked 'heads', 'skunk' or other potent
forms of cannabis. This means that 91% of the cannabis consumed in
Australia is 'heads' and other potent forms of cannabis.

Changing Patterns Of Cannabis Use

A major change in patterns of cannabis use among Australian adolescents and
young adults is that larger numbers of younger users use more potent forms
of cannabis at an earlier age. The 1998 NDS data show a strong trend
towards an earlier age of initiation among younger cannabis users. One in
five cannabis users (21%) born between 1940 and 1949 had initiated cannabis
use by age 18, compared to 43% of those born in 1950-59, 66% of those born
1960-69 and 78% of those born in 1970-79.

Earlier initiation of cannabis use increases the chances that these users
will become daily or nearly daily cannabis users. [37,38] This, in turn,
increases the risks of becoming dependent on cannabis and experiencing
problems as a result of their use. [29,37] Levels of consumption among some
adolescent cannabis users can be very high. For example, 40% of a sample of
NSW juvenile offenders reported smoking [40] or more 'cones' of cannabis a
week. [39]

The greater expense of cannabis heads also encourages regular users to
smoke them in waterpipes or 'bongs' in the belief that this maximises the
delivery of THC. In the 1998 NDS Survey just over half of all persons who
had used cannabis in the past year smoked 'heads' (57%) using bongs (56%).
Younger users were more likely than older users to prefer bongs or pipes to
joints and heads to leaf, with the trend reversed in older users.

All these changes in patterns of use - earlier initiation of cannabis use,
greater use of more potent cannabis products such as heads and the use of
waterpipes - have probably increased the amount of THC consumed by regular
cannabis users more than any speculative increase in the THC content of
cannabis plants.

What Are The Health Implications Of Any Increase In THC Content?

Proponents of the claim that THC content has increased regard it as
self-evident that it will increase the adverse health effects of cannabis
use. [1,3] Critics of the claim have countered that increased THC potency
may have little or no adverse effect because users are able to adjust or
'titrate' their dose of THC to achieve the desired state of intoxication.
[9,12] If users were able to titrate their dose of THC, as tobacco smokers
do with nicotine, [40] then the use of more potent cannabis products would
reduce the amount of cannabis material that was smoked. This would
marginally reduce the risks of developing respiratory diseases, the most
likely adverse health effect of regular cannabis smoking.

There is very little research on whether cannabis users are able to titrate
their dose of THC by modifying the amount of smoke that they inhale. Some
earlier studies suggested that they could [41] but more recent studies
report that cannabis users have limited ability to titrate their dose of
THC [42,43] so the issue has not yet been resolved.

If users do not titrate their dose of THC, the health effects of using more
potent cannabis products may depend upon the user's experience. Higher
average doses of THC will probably increase the risk of adverse
psychological effects of cannabis use (such as anxiety and panic attacks)
in first-time cannabis users [29] which might discourage further use of the
drug. [29] Among regular cannabis users, an increased dose of THC may
increase the risks of accidents among those who drive while intoxicated,
especially if cannabis use is combined with alcohol. [29,44] Research to
date has not confirmed that the use of cannabis alone impairs on-road
driving or increases the risks of motor vehicle accidents [44] but the use
of higher doses of THC may change this risk. The use of more potent
cannabis products may also increase the risk of regular cannabis users
developing dependence. [30] Regular use of higher potency cannabis by
persons with schizophrenia may exacerbate their illness. [19]

Implications For Policy

1. We need better evidence on the THC content of cannabis. Unsubstantiated
media assertions are a poor basis for public policy in any areas of public
concern.

2. We need analyses of the THC content of samples of cannabis consumed by
regular users. A systematic method of sampling should be used to ensure
that it is not only the exceptionally potent samples of cannabis that are
tested. Annual sampling of 100 to 200 samples of cannabis products would
provide indications of THC content of cannabis.

3. We also need studies of the extent to which regular cannabis users are
able to titrate their dose of THC. The assumed capacity of users to do so
is used by some to discount concerns about any increase in the use of more
potent forms of cannabis.

4. We need better assessments of the nature and extent of cannabis-related
problems among adolescents and young adults.

5. More attention needs to be paid to the problems that may arise from
cannabis use in health promotion efforts directed at young Australians.
This could be done as part of programs that aim to prevent tobacco use and
hazardous alcohol use among adolescents, [45] and as part of harm
minimisation efforts to encourage less risky patterns of cannabis use among
current users. [46]

Conclusions

The limited Australian data and data from the US and New Zealand suggest
that there has been a modest increase in the THC content of cannabis
plants. There probably has been an increase in the amount of THC consumed
by Australian cannabis users as a result of an increased use of more potent
cannabis products at an earlier age. Policies towards cannabis would be
better informed if data were collected on the THC content of Australian
cannabis products.

Acknowledgements

We would like to thank the following persons for their assistance: Keith
Bedford, Greg Chesher, Paul Christie, Louisa Degenhardt, Paul Dillon, Paul
Donkin, Linda Gowing, John Hannifin, Bill Lee, Simon Lenton, Michael
Lynskey, Paddy Mahony, Jane Maxwell, Vince Murtagh, Helen Poulsen, James
Robertson, Hari Singh, Greg Swensen and Paul Williams.

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