AbstractBackgroundThe benefits of low-dose ketamine for patients with treatment-resistant depression (TRD) and prominent suicidal ideation require further investigation. The effects of treatment refractoriness, the duration of the current depressive episode, and the number of prior antidepressant failures on ketamine efficacy also require clarification.MethodsWe recruited 84 outpatients with TRD and prominent suicidal ideation—defined as a score ≥4 on item 10 of the Montgomery–Åsberg Depression Rating Scale (MADRS)—and randomized them into 2 groups to receive 0.5 mg/kg ketamine or 0.045 mg/kg midazolam. We assessed depressive and suicidal symptoms prior to infusion; 240 minutes post infusion; and 2, 3, 5, 7, and 14 days post infusion.ResultsAccording to the MADRS scores, the antidepressant effect (P= .035) was significantly noted in the ketamine group up to 14 days than in the midazolam group. However, the antisuicidal effect of ketamine, as measured by the Columbia-Suicide Severity Rating Scale Ideation Severity Subscale (P= .040) and MADRS item 10 (P= .023), persisted only 5 days post infusion. Furthermore, the antidepressant and antisuicidal effects of ketamine infusion were noted particularly in patients whose current depressive episode lasted <24 months or whose number of failed antidepressants was ≤4.ConclusionsLow-dose ketamine infusion is a safe, tolerable, and effective treatment for patients with TRD and prominent suicidal ideation. Our study highlights the importance of timing; specifically, ketamine is more likely to achieve therapeutic response when the current depressive episode lasted <24 months and the number of failed antidepressants is ≤4.