Hmm You may want to talk to her as if your considering it, and give her ALOT of info and see how she reacts, that way if all hell breaks loose you can just say forget I brought it up. BUT the serious bad side to that is now if she does find out and she forbade you to use you are completely fucked.
Need to explain to her that there are only a few reasons there banned. 1. Idiots, then explain how idiots fuck up alcohol and how that was banned for a while 2. Sports, because yes its an unfair advantage and everyone would have to juice to keep the playing fields more level, and that would lead to unhealthy juicing just for the pure temporary competive edge, despite how badly it will fuck em over later from juicing in unsafe amounts. Kinda like taking 1/2 a bottle of pain killer before a fight, so you don't feel anything while it helps right then, it sucks for later, but pain killer such as aspirin in moderate doses ARE benificial to overall health.
And if you tell her I would suggest telling her what your doing is considered an extremely light cycle.
Also mabe talk about some of this legal prohormone garbage, and how its not as safe as smart doses of the real deal.
Or you can go really sacastic like, tell her "booyah the steroids just kicked in damn I'm mad powerfull all of a sudden. Time to go save the world from giant boulders or something really heavy." Now if she gets upset over somthing so obviously sarcastic she isn't going to handle the real news anywhere near how you would hope. If she just laughs or ignores you mabe you have a shot. Just keep bringing up the steroid thing mabe sarcastically, see how long it takes her to get conscerned the faster she gets concerned about it the worse off you will be when you tell her for real. Of course this doesn't always work, when asked where I'm going I often say things like I'm smuggling canadians into our country. No one has yet to bring up anything on the subject despite the seriousness of that situation and the detrimental effects it would have on our country
Either way please keep me or all of us posted on how it goes or what you decide to do. I know in a situation like that where, we were serious enough to move in together I would have a terrible time keeping it from her and would probably "attempt" to tell her.
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Hahaha My Girlfriend totally freaked out when I told I wanted to do a cycle. She was worried that we could not have kids anymore and that I will become aggressive and stuff like that. So I just said okay darling I wont do it Hahaha bastard that I am. 
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