Post-Traumatic Stress Disorder (PTSD) occurs after a severe and extraordinary stressor. Examples of these stressors include natural disasters such as large fires, and hurricanes, interpersonal violence such as muggings, rape and abuse; and large scale violences occuring in war or terrorist attacks.

There are two main stages of PTSD, the re-experiencing or intrusion phase, and the avoidance phase. In the re-experiencing phase, In the re-experiencing phase, the person with PTSD may be hypervigilant, feeling on-edge, and have their thoughts intruded upon by images such as hallucinations, illusions, nightmares, and other mental images. They can lose the ability to concentrate and find that sleep is difficult. They focus a great deal of the time on the stressor, to the point of compulsivity. Emotional ups and downs are common, and they are easily startled and upset by remote reminders of the stressor. The avoidance phase is marked by a general emotional numbing. The sufferer may minimize the significance of the stressor, forget the event, lose their interest in life, feel detached from others, show little in the way of mood changes, and may abuse drugs and alcohol. Untreated, PTSD may eventually lead to mood, panic, substance abuse, and phobia disorders.

There are three sub-types of PTSD. The first, acute, lasts less than three months; chronic, lasts more than three months; and delayed-onset begins more than six months after the stressor occurs. There is a very short-term disorder called Acute Stress Disorder which is similar to PTSD which is defined as lasting less than 4 weeks.

Decreasing the chances of developing PTSD is the main goal. The best way to do this is to hold a debriefing session as soon as possible after a critical incident has occurred. On going peer support groups can also be helpful for those who are temporarily overwhelmed by a stressor. For cases of active PTSD, individualized and/or group psychotherapy is generally indicated, although acute cases may end spontaneously. Medications can be utilized for some symptoms while in therapy, such as nightmares and flashbacks. The medication phenelzine (nardil) is commonly utilized. Tegretol may be utilized to control aggression and irritability, and propranolol and clonidine (catapres) can assist in decreasing the anger and startle responses.