Post-Traumatic Stress Disorder (commonly referred to by its acronym, PTSD) is a severe anxiety disorder that can develop after exposure to any event which results in psychological trauma. This event may involve the threat of death to oneself or to someone else, or to one's own or someone else's physical, sexual, or psychological integrity, overwhelming the individual's psychological defenses.

PTSD is a less frequent and more enduring consequence of psychological trauma than the more frequently seen acute stress response. PTSD has also been recognized in the past as railway spine, stress syndrome, shell shock, battle fatigue, traumatic war neurosis, or post-traumatic stress syndrome.

Diagnostic symptoms include re-experiencing original trauma(s), by means of flashbacks or nightmares; avoidance of stimuli associated with the trauma; and increased arousal, such as difficulty falling or staying asleep, anger, and hyper vigilance. Formal diagnostic criteria (both DSM-IV and ICD-9) require that the symptoms last more than one month and cause significant impairment in social, occupational, or other important areas of functioning (e.g. problems with work and/or relationships).

A study conducted at the Universitá di Firenze in Italy finds there is positive indications cannabis could help treat PTSD: "Consolidation refers to item stabilization in long-term memory. Retrieval renders a consolidated memory sensitive, and a "reconsolidation" process has been hypothesized to keep the original memory persistent. Some authors could not detect this phenomenon. Here we show that retrieved contextual fear memory is vulnerable to amnesic treatments and that the amygdala is critically involved. Cholinergic and histaminergic systems seem to modulate only consolidation, whereas cannabinoids are involved in both consolidation and reactivation. The liability of retrieved memory affords opportunities to treat disorders such as phobias, post-traumatic stress, or chronic pain, and these results help searching for appropriate therapeutic targets."