Ketamine for Relapse Prevention in Recurrent Depressive Disorder

Inclusion Criteria: * ≥18 years old * Hamilton Rating Scale for Depression, 24-item (HRSD-24) score of ≥21 * Voluntary admission for treatment of acute depressive episode * Meet DSM-IV criteria for recurrent depressive disorder (RDD): ≥2 previous depressive episodes with at least 2-months(consecutive) subthreshold or no symptoms in between PLUS(to enrich the sample for those at high risk for relapse) must also have experienced ≥3 major depressive episodes(including index episode) within the previous 2 years For the randomised pilot trial, RDD patients must have: * received antidepressant treatment for the acute depressive episode(pharmacological, psychotherapeutic or multidisciplinary) * ≥60% decrease from baseline HRSD-24 score and score ≤16 * Standardised Mini-Mental State Examination (sMMSE) score of ≥24 * able to provide informed consent Exclusion Criteria: * Current involuntary admission * Medical condition rendering unfit for ketamine/midazolam * Active suicidal intention * Dementia * History of Axis 1 diagnosis other than RDD * Electroconvulsive therapy (ECT) for treatment of current depressive episode * Alcohol/substance abuse in previous six months * Pregnancy or inability to confirm use of adequate contraception during the trial