Additional Study Information: If you're experiencing an acute psychiatric emergency, please stop negotiating this website and call 911 or go to the nearest emergency room. This website is not designed to address an acute psychiatric emergency. Purpose: To find out whether intranasal ketamine, when added to standard antidepressant treatment, improves the outcome of treatment for people with depression and suicidal thinking. All patients in the study will receive standard treatment for depression. Some will receive intranasal ketamine and others will receive inactive placebo. Placebo nasal spray looks the same as the ketamine nasal spray and is given the same way but has no active drug in it. Study procedures: At most of the study visits, you will take the ketamine or placebo nasal spray (under supervision) and you will be given some scales to rate how you feel. Length of study: You will be in the study for about 13 weeks. Screening will take place in the emergency room or inpatient unit at the study center and should be completed within 24 to 48 hours before you start taking the study drug. If you are eligible to continue, you will take the ketamine or placebo while staying in an inpatient unit for approximately 5 days and then in the study outpatient treatment center until Day 25. After Day 25, you will visit the study clinic weekly for the first 4 weeks and then every 2 or 3 weeks until the end of the study (Day 90). Benefits: Taking part in this study may help symptoms of depression, but this is not guaranteed to happen. There may be no benefit to you. Your symptoms of depression or suicidal feelings may worsen. Risks: There are risks and discomforts associated with participating in this study. Ketamine is a general anesthetic at higher doses. In this study it is used at lower doses that are still likely to cause some grogginess and feelings of detachment from one's surroundings among other side effects. This study is completely voluntary.