Mortality among opioid-dependent users varies across countries and populations. Treatment is clearly protective against mortality even in non-randomized observational studies. Study characteristics predict mortality levels; these should be taken into account in future studies.

The prevalence of psychosis among the current sample of methamphetamine users was 11 times higher than among the general population in Australia. Dependent methamphetamine users are a particularly high-risk group for psychosis.

Increased potency has been observed in some countries, but there is enormous variation between samples, meaning that cannabis users may be exposed to greater variation in a single year than over years or decades. Claims made in the public domain about a 20- or 30-fold increase in cannabis potency and about the adverse mental health effects of cannabis contamination are not supported currently by the evidence. Systematic scientific testing of cannabis is needed to monitor current and ongoing trends in cannabis potency, and to determine whether cannabis is contaminated. Additionally, more research is needed to determine whether increased potency and contamination translates to harm for users, who need to be provided with accurate and credible information to prevent and reduce harms associated with cannabis use.

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