The THC content of cannabis in Australia: evidence and implications.

OBJECTIVE: To examine evidence on three claims that: 1) the THC content of Australian cannabis plants has increased up to 30 times; 2) problems experienced by cannabis users have increased in Australia in recent years; and 3) an increase in THC content is the most likely explanation of any increase in cannabis-related problems.

METHODS: These claims were assessed by examining data: 1) on THC potency in Australia, the United States and New Zealand; 2) on cannabis-related problems; and 3) from the 1998 National Drug Strategy Household Survey on patterns of cannabis use.

RESULTS: 1) Published data do not show a 30-fold increase in THC potency of cannabis but show a more modest increase in the US. 2) There is suggestive evidence of an increase in cannabis-related problems among people seeking treatment for alcohol and drug problems, juvenile offenders and young adults with psychosis. 3) There are two other more plausible explanations for these reportedly higher rates of cannabis-related problems among adolescents and young adults: (i) more potent forms of cannabis ('heads') are more widely used; and (ii) cannabis users are initiating cannabis at an earlier age, thereby increasing the prevalence of harmful patterns of use.

CONCLUSIONS: There has probably been a modest increase in the THC content of cannabis, but changing patterns of cannabis use have probably made a larger contribution to any increase in rates of cannabis-related problems among young Australian adults.

IMPLICATIONS: Better data on the THC content of cannabis, the extent of cannabis-related problems and the ability of users to titrate the dose of cannabis would contribute to more informed debate.