I stumbled across this while Googling, it's easy to read and answers a lot of questions people have about this medicine including how it works, side effects and things you shouldn't take at the same time.
drugs.com/slideshow/prednis...
Some other useful links about Pred and why it causes side effects include:
onlinelibrary.wiley.com/doi...
and this
ncbi.nlm.nih.gov/pmc/articl...
Thanks to PMRPro for the links