Pragmatic, randomised, controlled, parallel-group, pilot trial. Trial participants will be patients admitted to St Patrick's University Hospital for treatment of a depressive episode. The investigators aim to recruit up to 20 participants who will be eligible for this study and randomly allocate 10 patients to each group. The participants will undergo usual inpatient care as prescribed by their treating team for the index acute depressive episode. Both participants and assessors will be blind to treatment allocation. Consented participants will be randomly allocated in a 1:1 ratio to a four week course of either once-weekly ketamine or midazolam infusions. Block randomisation will be independently performed. Physical, psychotomimetic and cognitive outcomes will be monitored before, during and after infusions. Blood samples will be taken at four time-points in the first infusion session and before the final infusion for neuroplasticity biomarker studies. Both groups will continue treatment as usual. Participates will also be followed up over a three month period.
