Can buspar (buspirone) be taken prn?

No: No. For it to work you need to take it daily.

Buspirone or Buspar (buspirone): Buspirone needs to be taken regularly every 6-8 hours in an amount enough to reduce anxiety. It does not work prn because it needs to build a level in your nervous system to act appropriately. It is very effective taking up to 2-3 days to stabilize in your system. The usual doses are 10-20 mg three times a day with meals. It is a great alternative to sedatives, and also acts as a mood stabilizer.