Ketamine is a robust and rapid-acting antidepressant, albeit its effect is transient. Intravenous ketamine infusion in treatment resistant depression results in improved mood within 4 hours reaching the peak at 24 hours. The effect is diminished at 7 days, and most patients relapse within 10 days, although for a significant minority the improvement may last 30 days and longer. The main challenge with ketamine treatment is what to do when the anti-depressive action expires. The maintenance therapy with ketamine (from twice a week to once in two weeks) appears to be a promising option, although the evidence to firmly recommend it is insufficient. Ketamine may also decrease suicidal thoughts for up to three days after the injection. Ketamine may be effective for bipolar depression, but the data on its use is scarce.