The influence of substance use on the effectiveness of antipsychotic medication: a prospective, pragmatic study

Background: Psychosis is associated with a high prevalence of substance use, leading to worsened prognosis. Less is known about how comorbid substance abuse may influence the effectiveness of antipsychotic medications. The aim of this study was to compare the effectiveness of second generation antipsychotics in patients with psychosis with and without substance use. Methods: All patients (n = 226) were aged >18 years old had symptom level scores of ≥4 on selected psychosis items on the Positive and Negative Syndrome Scale and met ICD-10 diagnostic criteria for psychosis. Information on substance use was collected based on the Clinician Drug Use Scale. Patients were grouped at baseline according to the presence of substance use, medication history and diagnosis group. Clinical symptoms at baseline and changes at follow-up were assessed with the PANSS. Results: At baseline about 30% of the patients used substances, most frequently cannabis followed by methamphetamine. About half (47%) of the patients had no prior exposure to antipsychotic medication at inclusion. Patients who had consumed substances showed no substantial differences in the PANSS score reduction as a result of antipsychotic medication compared to patients without substance. There were, however, some group differences in relation to pattern of change that were influenced by medication history. Substance use was found to be related to stronger reduction of positive symptoms from week 4 to week 27. Conclusion: Substance use alone did not influence antipsychotic effectiveness in this sample of patients with psychosis.