Objective: There is conflicting data on drug abuse and outcome in severe mental illness. This study aims to investigate if the amount of illicit psychoactive drug use is related to symptom load or premorbid functioning across diagnosis in patients with severe mental illness.
Method: Symptom load, sociodemographic status, premorbid functioning and the level of use of illicit psychoactive drugs were assessed in 423 subjects with schizophrenia or bipolar disorder in a cross-sectional study.
Results: High amount of illicit drug use was associated with poorer premorbid academic functioning. In schizophrenia, there was a significant positive association between amount of drug use and severity of psychiatric symptoms. The association between symptom load and drug use was significant after controlling for premorbid functioning.
Conclusion: The results suggest a direct association between the quantity of current drug use and more severe symptoms in schizophrenia. Poor premorbid functioning was related to high amount of use, but did not explain the difference in symptom load.