Although these data illuminate characteristics of substance use in psychotic disorders in a large, multi-ethnic population, further study is required to better understand the nature of the comorbidity between psychotic disorders and substance dependence. The first step is to specifically evaluate comorbid substance dependence in individuals with severe psychotic illness, rather than individual measures of use as assessed in this study. In addition, the validity and reliability of this series of questions has not been established in any population. However, this series of questions was extracted from the DIGS, a standardized instrument with high test/retest validity and reliability. A further limitation of this study is that it is not a population survey. Because the individuals were not randomly sampled, there may be biases in the dataset that limit extrapolation of the rates of substance use to the general populations of both cases with severe psychotic illness and controls without a personal or family history of bipolar disorder or schizophrenia. However, the sample was not selected for substance use, so therefore the odds ratios of substance use for cases versus controls should be accurate.
