NDARC Technical Report No. 74 (1999)


A number of commentators in the alcohol and other drugs field have recently claimed that the THC content of cannabis used in Australia has increased between 10 and 30 times over the past two decades. This has raised understandable queries about its role in a possible increase in the adverse health and psychological effects of cannabis use experienced by young people.

There is a need to distinguish two different interpretations of this claim: (i) that the average THC content of cannabis plants has increased, and (ii) that the average THC content of cannabis products as consumed by Australian users has increased by 10-30 times. We examine evidence on each claim and discuss the implications for harm.

Has the average THC content of cannabis plants increased?
The THC content of Australian cannabis products has not been systematically tested by any Australian police force over the period in which average THC content has been claimed to have increased, as there is no legislative reason for doing so. There is therefore no Australian data to support the claim that there has been a 10 to 30-fold increase in average THC content of cannabis in Australia.

The USA is the only country that has regularly collected data on the THC content of cannabis plants over the past several decades. Claims that this data indicated that the THC content of marijuana in the USA had increased between three to seven-fold from the early 1970s to the mid 1980s have been challenged by data from independent laboratories, and because such claims relied on the assumption that the samples from the middle 1970s were representative of cannabis consumed at that time. More recent data have failed to show a 10-30 fold increase in the THC content of seizures between 1984 and 1998. At most this series shows a small increase in THC content from 3.3% in 1980 to 4.4% in 1998.

The New Zealand government has also intermittently tested the THC content of cannabis samples over the past two decades. Samples of hydroponically grown cannabis tested in a NZ survey typically contained 6-8% THC, with an occasionally higher sample.

There are two more plausible alternative explanations for an upsurge in anecdotally reported adverse effects of cannabis use: changing cannabis markets have increased the availability of more potent forms of cannabis; and there have been changes in the patterns of cannabis use among Australian cannabis users that increase the prevalence of harmful use.

Changing Cannabis Markets
Over the past two decades a large scale illicit cannabis industry has developed in Australia to meet the demand for cannabis products among a growing number of cannabis users. It has been estimated that daily and weekly cannabis users, who prefer the more potent forms of cannabis, account for 80% of cannabis consumed. Any increase in the number of regular cannabis users that may have occurred in recent decades may have increased the demand for and availability of more potent forms of cannabis. Any such increase in the availability of more potent forms of cannabis would have increased the amount of THC consumed by heavier cannabis users without there having been any increase in the average THC content of cannabis plants.

It is also a plausible hypothesis that the supply of more potent cannabis products has been encouraged by the success of domestic law enforcement in detecting and destroying large-scale cannabis plantations, creating an incentive for illicit suppliers of cannabis to grow small numbers of cannabis plants with a higher THC content (e.g., hydroponic cultivation).

Changing Patterns of Cannabis Use
Survey data suggest that in the 1990s young Australians have probably initiated cannabis use at an earlier age than was the case in the 1980s. The lifetime prevalence of cannabis use in Australia and the United States, particularly among adolescents, has increased, after a decline in the 1980s and early 1990s. Earlier initiation of cannabis use increases the chances that users will become daily or nearly daily cannabis users, and increases the likelihood that they will become dependent on cannabis and experience adverse personal and social consequences as a result of their use. Regular cannabis use makes users tolerant to the effects of THC, encouraging the use of more potent cannabis preparations and the use of waterpipes or "bongs" in the belief that it maximises the delivery of THC. Population based data reveal that "heads" are the most commonly used form of cannabis in Australia, while cannabis is most frequently smoked in waterpipes or "bongs".

All these changes in patterns of use – earlier initiation of cannabis use, more regular use of more potent cannabis products and greater use of waterpipes – have probably played a greater role in increasing the amount of THC consumed by regular cannabis users than any increase in the average THC content of cannabis plants. And it is these patterns of use that may explain apparent increases in numbers of adolescents and young adults experiencing problems with their cannabis use.
What are the health implications of any increase in THC content?

A major concern raised by the claimed increase in the THC content of cannabis has been that it will increase the adverse health and psychological risks of cannabis. Research suggests that unlike nicotine smokers, cannabis users have only a limited ability to titrate their dose. In this case, the effect of using more potent cannabis products will depend upon the type of health effect in question, and the user’s experience with cannabis. Higher average doses of THC will probably increase the risk of adverse psychological effects of cannabis use, an effect likely to be most obvious among naive or first time cannabis users. This effect may discourage further experimentation with the drug among these users. Risks of increased THC exposure among regular cannabis users are less certain. They possibly include an increased risk of accidents among those who drive while intoxicated, especially if cannabis use is combined with alcohol, and an increased risk of regular cannabis users developing dependence. If the THC content of the most commonly used Australian cannabis products has increased, the net adverse effects of cannabis use may have marginally increased. Respiratory risks may be marginally decreased if cannabis smokers are able to titrate their doses of THC.

A first priority should be to collect data on the THC content of samples of cannabis consumed by regular users in Australia. A second priority should be a study of the extent to which regular cannabis users are able to titrate their dose of THC. If users perform this task poorly, then this information needs to be given to users and potential users.

A potential legislative strategy is to impose higher penalties for the cultivation and sale of cannabis products with THC content above some statutory level. This may provide incentives for illicit manufacturers to supply the less potent forms of cannabis products.

A more contentious issue is the type of health education advice that should be given to young people about the health risks of using more potent forms of cannabis. We do not need to wait until we have better data on THC content before we educate young people about: the risks of early initiation of cannabis (and other licit and illicit drug use); the greater potential for dependence if they use more potent forms of cannabis; and the respiratory risks of smoking cannabis in bongs (Hall, 1998).



Wayne Hall, Wendy Swift
Date Commenced
10 Sep 1999
Resource Type
Technical Reports