Hi tigger67: You have nothing to worry about. The amount that they give you in an epidural, trigger point, or facet injection isn't enough to cause any problems. I was also told that the amount, in the injection, is less than what your body produces in a day. I've asked this question many times and every Dr including my mineral metabolism Dr said the same thing.
Just to make sure as your Dr. I've had many epidurals and trigger point injections plus I've taken the Medrol dose pack, and I didn't have to worry about it. There is a limit to how much cortisone you can get in a year, but your Dr should tell you that. If you're getting an epidural, for your back, it's usually a series of 3 injections. If you are really concerned, ask the Dr who's giving the injection how much they're using and how often, and then give that info to you osteo Dr and see what they say.
Long term use of cortisone can cause osteo, so watch that.