What you tell them and how is pretty much dependent on your personality. I find humor is a great equalizer. If I was in that spot your in, I would make a joke out of it. That breaks the ice and ruins the fun any one might have in talking about it.

So it would go some thing like this..

Hey Sinking, why were you in the hospital for a month?

"detox! damn never knew heroin was that hard to get off of" (followed by a smile)

Typical response to me then would be a laugh and then may be asked again,

Very funny, but really what was up?

" just some boring medical reasons, I'm sure the rumors are more exciting then the truth which I'd rather not talk about"

But that would be me in my current work environment.

I agree with the others, the least they know the better