Patients with PTSD, and frequent nightmares, and mild-moderate suicidal ideation, who are already taking a Selective Serotonin Reuptake Inhibitor (SSRI) or Serotonin Norepinephrine Reuptake Inhibitor (SNRI) will be randomized to either prazosin or placebo. The investigators hypothesize that patients receiving prazosin will have a greater reduction in suicidal ideation.