Not really. Here's the problem with Xanax (alprazolam)- it has the potential for being addictive, and once you are addicted to it, then you have another separate problem besides your PTSD (post traumatic stress disorder). Xanax and the rest of the benzodiazepines are only meant to be taken short term to deal with acute anxiety symptoms. Once the acute anxiety symptoms are addressed, then the benzodiazepine should be tapered and discontinued.
If you use benzodiazepines for longer than a few weeks, then you risk being addicted to it, and are subjected to the numerous other side effects, such as sedation, dizziness, and confusion. The potential for addiction are due to Xanax causing a 'high,' feeling euphoria from taking it. However, these euphoric effects wear off, and you need more and more of the Xanax in order to feel this same euphoria. Soon, you are taking huge doses of the Xanax, and you then become addicted to it.
So the answer is no...Xanax is not good for PTSD. If you have PTSD, you are better off not even starting it. There are other effective treatments for PTSD that have less side effects, and do not have the addiction potential of Xanax.