Prazosin
"I've had ongoing PTSD for the past 15 years, including nightmares almost every night. My neurologist finally started me on prazosin to see if it would help me get any sleep. I had to go up to 4mg to make the nightmares stop. I tried to go back down to 3mg but had nightmares so went back to 4mg. 

After 2 months at 4mg I'm back to not sleeping well and having nightmares 4-5 nights a week. I'm concerned that I will continue to build up a tolerance and it will become less effective. 

I did have one incident where I got up in the middle of the night and passed out, remaining unconscious for over 40 minutes. I have to really watch out for low blood pressure when taking any BP altering meds like this. A salty snack before bed seems to help."