Estimating the number of regular and dependent methamphetamine users in Australia, 2002–2014

We thank Dietze and colleagues for their letter and offer the following responses.

First, our published estimates1 were drawn from two multiplier–benchmark pairs and produced very similar results.

Second, the letter describes the sample from which our multipliers were drawn2 as “predominantly injecting”. In fact, 64% of our sample predominantly injected methamphetamine, compared with 52% of the sample used by the authors.

Third, multiplier estimates require that the multiplier matches the benchmark data. The estimates published in our article comply with this requirement because the questions were carefully phrased2 to match the Alcohol and Other Drug Treatment Services National Minimum Data Set (AODTS-NMDS). Dietze and colleagues’ multipliers do not match these data as they are based on a question assessing the use of “any health and support services” for methamphetamine, in addition to specialised alcohol and other drug treatment. This would produce an artificially low multiplier.

Fourth, most of the possible reasons for variations in the treatment multiplier over time, as discussed in our article, would produce a larger multiplier than the one we used.

Fifth, the authors’ treatment multiplier for dependent use (3.6) is much lower than ours (5.5), implying that methamphetamine treatment coverage increased 53% between 2004 and 2009. This is implausible as the number of treatment agencies in Australia was stable over this time.3

Sixth, since we do not have enough information about the methods used by Dietze and colleagues, we could not replicate their calculations. For example, applying their multipliers to the number of AODTS-NMDS amphetamine treatment admissions in Australia for the year 2013–14 produced estimates of 162 000 regular users, of whom 104 000 were dependent.

Last, their estimates for dependent use are implausibly low in light of other data. The estimate of dependent users (43 322) is not much greater than the total number of amphetamine treatment episodes in Australia in 2013–14 (28 919). If this were true, it would mean that each dependent methamphetamine user had 0.67 treatment episodes per year — an implausibly high rate that is inconsistent with their own treatment multiplier of 3.6, which implies 0.28 episodes per person per year.

The estimate by Dietze and colleagues would mean that the scale of dependent methamphetamine use in Australia has almost halved since 2003–04 (73 000). Their estimate of 43 322 is also less than half of the 2007 household survey estimate of 97 000,4 which, like all surveys, is likely to underestimate the prevalence of problematic use. In addition, the incidence and frequency of methamphetamine use have increased in the latest household survey.5