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