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.
More (controlled) clinical trials are required.
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.