Significance Statement
TRD, depressive, ideation
Low-dose ketamine infusion had rapid antidepressant and antisuicidal effects among patients with TRD and prominent suicidal ideation. In addition, the antidepressant effect of ketamine persisted for up to 2 weeks, but the antisuicidal effect lasted only 5 days. Furthermore, timing of ketamine treatment is crucial; specifically, patients with current depressive episodes that have persisted <24 months or ≤4 failed antidepressant treatments may receive the greatest benefits from low-dose ketamine infusion.