There are plenty of reasons not to tell a girlfriend especially a younger one - women talk too much... but as for your wife, it's way too hard to hide, and it's better when it comes out.
My wife asked me point blank "that acne on your back, that's not like you - what is that, a steroid thing?" I was shocked and denied it. Then the next day I explained it all to her and took care to explain why I didn't tell her - how there are a lot of misconceptions that oral is safer than injection when in fact it is just the opposite, etc.
It's just fine now. She is probably under the impression that I could kick some ass now if need be, what with all the road-raging idiots out there (how wrong she is, hehe).
Actually the worst part of it was her comparing me to Victor Mature (wasn't he from like the 1940's?) when I did a back pose. I mean, Victor Mature? How old is my wife anyway?