I hurt my neck and needed surgery and had all sorts of cortisol shots and even oral prednisone for a couple weeks. I never noticed any effect or reduction in strength. However, slightly more long-term use of prenisone (or continuous cortisol shots) should impact your gains because it is a glucose sparing - stress hormone that will cause mild muscle loss. It will also cause fat redistribution to your gut with prolonged use. Kids with muscular dystrophy that are on extended prednisone, or severe asthmatics or people with other inflammatory diseases, show muscle loss and fat redistribution. Medically well known side effect. One shot will not hurt you though, or it will only have a very mild and temporary effect. Will totally eff up your sleep though (makes you feel energized!!!)