Ketamine may be effective, but is it useful for 'immediate' prevention? I suppose this depends on what you mean by 'immediate'. Someone already posted a link here suggesting a significant number of suicides are impulsive, and many people have planned suicides without their friends or family having any idea.

I can't see how in many of these situations a controlled drug is going to help. We could only administer ketamine to those who exhibit depression or suicidal tendencies, but sadly these signs are not always visible or picked up.