To examine trends in hospital separations related to "drug-induced" psychosis for cannabis and methamphetamine, in the context of patterns of cannabis and methamphetamine use in the Australian population.

Analysis of prospectively collected data from the National Hospital Morbidity Database on hospital separations primarily attributed to drug-induced psychosis (July 1993 - June 2004), and specifically for cannabis and amphetamines (1999-2004). Calculation of Australian population-adjusted rates of drug-induced psychosis hospital separations using estimated resident population data from the Australian Bureau of Statistics (at 30 June each year) and data on cannabis and methamphetamine use from the 2004 National Drug Strategy Household Survey.

Number of hospital separations due to drug-induced psychosis, and standardised (age-specific) rates per million population and per million users.

There have been notable increases in hospital separations due to drug-induced psychosis, which appear to have been driven by amphetamine-related rather than cannabis-related episodes. The rate of hospital separations was higher for amphetamine users than for cannabis users in all age groups, and the rate increased among older amphetamine users.

The risk of hospitalisation for a drug-induced psychotic episode associated with amphetamine use appears to be greater than that for cannabis use in all age groups.