There is evidence that cannabis/cannabinoids can be used to treat PTSD and to prevent its development. To date, most evidence points towards THC and CBD and their function to speed up the extinction of fear memories. More research is required which cannabinoids have a therapeutic indication for PTSD and via which mechanisms.

Literature Discussion: 

Similar to chronic stress, people with PTSD have 15-20% lower CB1 levels and more than 50% reduced Anandamide levels (Neumeister et al., 2013) which may form a mechanistic insight in the development of PTSD and/or depression.

Literature:

Neumeister, A., Normandin, M.D., Pietrzak, R.H., Piomelli, D., Zheng, M.Q., Gujarro-Anton, A., Potenza, M.N., Bailey, C.R., Lin, S.F., Najafzadeh, S., et al. (2013). Elevated brain cannabinoid CB1 receptor availability in post-traumatic stress disorder: a positron emission tomography study. Mol. Psychiatry 18, 1034–1040.

Clinical Trials: 

CBD and THC were found to speed up/enhance fear extinction which should help prevent the development of PTSD (Das et al., 2013; Rabinak et al., 2013). In one open clinical trial the use of cannabis was found to provide significant relief of PTSD symptoms in most patients.

http://www.cannabis-med.org/studies/ww_en_db_study_show.php?s_id=481&&search_pattern=posttraumatic

More (controlled) clinical trials are required.

Literature:

Das, R.K., Kamboj, S.K., Ramadas, M., Yogan, K., Gupta, V., Redman, E., Curran, H.V., and Morgan, C.J.A. (2013). Cannabidiol enhances consolidation of explicit fear extinction in humans. Psychopharmacology (Berl.) 226, 781–792.

Rabinak, C.A., Angstadt, M., Sripada, C.S., Abelson, J.L., Liberzon, I., Milad, M.R., and Phan, K.L. (2013). cannabinoid facilitation of fear extinction memory recall in humans. Neuropharmacology 64, 396–402.  

Wiki Entry: 
Receptors: 
Endocannabinoids: 
Phytocannabinoids: 
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Alternative Names: 
Post Traumatic Stress Disorder