Substance induced psychosis and Bradford-Hill Criteria for Causation

Substance induced psychosis is unusual among the categories described in systems of psychiatric classification in specifying a cause of the condition. However, despite a clear association, the strength of the causative inference is by no means clear. In this article, the criteria for causation proposed by the pioneering epidemiologist Sir Austin Bradford-Hill are applied to what is known about the relationship between substance use, in particular, the use of cannabis and stimulant drugs, and psychotic illness, in order to examine the validity of the diagnostic category of substance induced psychosis, and whether a causative inference should be drawn in making the diagnosis in many cases. The article will also discuss recent changes to the status of substance related psychoses in criminal law in New South Wales.