I’ve been told by doctors that Xanax taken PRN does not work well for panic attacks because, even if taken at the onset of the attack, the attack is pretty much over by the time the drug starts to work. I have been told that it is effective taken PRN for generalized anxiety or social anxiety, because the drug takes effect long before the symptoms of these run their course (perhaps not until one gets back home).
I do not promote the use of Klonopin or Xanax, but taken on a regular schedule, either of these has been effective in preventing panic attacks in me. I take Klonopin over Xanax for two reasons: 1) you don’t feel Klonopin taking effect or wearing off – no inter-dose withdrawal; and 2) it is supposed to be easier to taper from. I’m not so sure about the latter.