Police pepper spray immunity training?

Being "pepper sprayed" is an uncomfortable experience. It is intended to be so that the officer can understand what it is that he is doing. I don't think that "painkillers" would reduce the discomfort. If you are considering using illegal drugs to avoid feeling the very transient effects of being "pepper sprayed", I'd encourage you to seek another line of work. I was "pepper sprayed" in my eyes during my training. I wouldn't want to do it again, but it was really not all that bad.