In this article by Raskind et al. in the American Journal of Psychiatry (Sep. 2013), combat veterans with PTSD were given prazosin or placebo in a 15 week randomized study:
The prazosin was substantially beneficial for PTSD symptoms. Dosing was on average about 5 mg in the mid-morning + 15 mg at bedtime.
There was a large, clinically relevant, sustained difference from placebo, and the prazosin was well-tolerated.
The experience from this study differs from my own experience prescribing this drug, in that the doses were much higher. Also, they used daytime doses, which I think shows acknowledgment that prazosin is not just useful for nightmares, but for all symptoms of PTSD, day and night. I had previously prescribed in the 1-2 mg range, but I see there is a lot of room to move to higher doses, which could be much more effective without major safety or side-effect risks.