I've been juicing for almost 15 years and I've had several relationships during that time. I've never told any of my gf's and I never will. A few have suspected but I was never caught. I generally don't advise telling your gf. This one doesn't sound like she has an open mind. Rumors and myths are powerful and hard to overcome. Even if she reluctantly accepts it, it is likely that when you argue or get mad, she'll blame it on "roid rage" and want you to quit. Also, if/when you break up, you ain't friends no more, and she has knowledge about illegal activities that you definitely don't want her to have. Better to not even open that door in my opinion.