Optimal drug, dose and duration of antipsychotic therapy following amphetamine and cannabis-induced psychosis: A systematic review.

Hall E.; Ratnayake N.; Waldmann J.; Daglish M.; Scott J.; Connor J.; Parker S. · The Australian and New Zealand journal of psychiatry · 2026

**Background & Methods** This systematic review examined randomized controlled trials and observational studies investigating antipsychotic medications for treating amphetamine- and cannabis-induced psychoses. The review included 11 studies on amphetamine-induced psychosis (724 participants) and 4 studies on cannabis-induced psychosis (1,872 participants), with quality appraisal using Cochrane Risk-of-Bias 2 and Joanna Briggs Institute tools. **Key Findings** • Multiple antipsychotics (aripiprazole, haloperidol, olanzapine, paliperidone, quetiapine, and risperidone) demonstrated effectiveness in reducing psychotic symptoms in amphetamine-induced psychosis, but no single regimen demonstrated superiority over others. • For cannabis-induced psychosis, risperidone, haloperidol, and olanzapine were associated with reduced psychotic symptoms, though comparative efficacy data between agents remained limited. • Critically, no relapse prevention data was available following antipsychotic treatment for amphetamine-induced psychosis, and only limited evidence existed regarding relapse prevention following cannabis-induced psychosis treatment, creating significant gaps in clinical guidance. **Dosage & Administration** Not reported. The review did not specify optimal antipsychotic doses, routes of administration, or treatment duration protocols for either condition. **Safety & Adverse Effects** Not reported. The review did not address adverse effect profiles or safety comparisons between different antipsychotic regimens in these substance-induced psychosis populations. **Evidence Quality** This systematic review identified substantial evidence gaps limiting clinical applicability. Primary limitations include: (1) absence of follow-up data post-treatment cessation, preventing assessment of relapse rates or optimal treatment duration; (2) heterogeneity across included studies limiting meta-analytic synthesis, necessitating narrative summary; (3) small sample sizes in individual studies and limited number of cannabis-induced psychosis trials (n=4); and (4) inability to establish optimal dosing strategies or comparative effectiveness hierarchies. The lack of long-term follow-up studies represents a critical evidence gap, as clinicians cannot determine whether brief antipsychotic courses prevent relapse or if prolonged therapy is necessary. These limitations mean current evidence supports antipsychotic efficacy for acute symptom reduction but provides insufficient guidance for evidence-based treatment protocols regarding drug selection, dosing, and duration in either amphetamine- or cannabis-induced psychosis.

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