Background: The management of acute agitation manifesting in patients with schizophrenia or bipolar disorder requires swift pharmacological intervention to provide rapid symptomatic relief and prevent escalation to aggression and violence. Antipsychotic medications are widely used in this setting and the availability of an inhaled formulation with deep lung absorption of the antipsychotic loxapine has the potential to deliver a faster onset of therapeutic effect than the available intramuscular formulations of antipsychotics. Methods: The efficacy of inhaled loxapine and the alternative antipsychotic aripiprazole delivered via intramuscular (IM) injection will be compared in the Phase IIIb PLACID study. Adults (18-65 years) with a confirmed diagnosis of schizophrenia or bipolar I disorder presenting with acute agitation will be randomly assigned to open-label treatment in a 1:1 ratio. Clinical evaluation will be conducted by raters blinded to treatment assignment. The primary efficacy endpoint is time to response (defined as a Clinical Global Impression of Improvement [CGI-I] score of 1 [very much improved] or 2 [much improved]). Secondary endpoints will include the percentage of responders at different time points after dosing; the proportion of patients who receive 1 or 2 doses of study drug; time to second dose; time to rescue medication; satisfaction with study drug (evaluated using Item 14 of the Treatment Satisfaction Questionnaire for Medication); and safety and tolerability. Approximately 360 patients will be recruited with an interim analysis conducted once 180 patients have completed the study to decide whether to stop for futility or continue with or without an increase in the sample size up to additional 288 patients. Discussion: The PLACID trial will assess the efficacy and safety of inhaled loxapine with deep lung absorption compared with the IM antipsychotic, aripiprazole, in acutely agitated patients with schizophrenia or bipolar disorder. In the event that the median time to response of inhaled loxapine is significantly shorter than that of the intramuscular aripiprazole, the PLACID study has the potential to support the inhaled antipsychotic therapy as the standard of care in this setting.