Illicit drug use is common in individuals with schizophrenia, and it has been suspected that many individuals under-report their use of substances, leading to significant barriers to treatment. This study sought to examine the degree to which individuals with schizophrenia disclose their use of drugs on self-rated assessments, compared to laboratory assays, and to determine the contributors of under-reported drug use in this population.

A total of 1042 individuals with schizophrenia who participated in screening/baseline procedures for the Clinical Antipsychotic Trials of Intervention Effectiveness (CATIE) completed self-rated assessments of substance use and laboratory drug testing. Laboratory tests assayed cannabis, cocaine and methamphetamine use; the procedures included radioimmunoassay (RIA) and urine drug screens.

A significant proportion of participants tested positive for drug use on laboratory measures (n = 397; 38%), and more than half (n = 229; 58%) did not report using these drugs. Logistic regression models confirmed that patients who were most likely to conceal their use tended to be older, and presented with greater neurocognitive deficits. Patients who accurately reported drug use tended to have greater involvement with the criminal justice system. Illness severity and psychopathology were not associated with whether patients disclosed drug use.

Rates of under-reported drug use are considerable among individuals with schizophrenia when compared to laboratory assays, and the exclusive reliance on self-rated assessments should be used with caution. Patients who under-report their drug use are more likely to manifest neurocognitive deficits, which could be improved by interventions attempting to optimize treatment.