Its her or the AAS then its not a relationship worth having. Talk to her about it and inform her a bit but dont try to fill her brain with ridiculous stuff she wont get.
I tell my GF everything i use, give a basic idea what it does, possible side, how i handle the sides. I recently told her in a few months i may go to AAS. She was a little iffy, not cause size or anything else, but rather side effects and future health. I was honest with her and she understands it wont last forever.
Tell her. Then youll find out if you have a girl that truely understands you and cares for you.
I wasnt going to post here but I find myself really in disagreement with this statement.
If its a serious relationship, which is sounds like it is, then it is very much her right to be concerned or possibly object. Just because she is against it does not mean she does not "understand" him or is not "worth it". If a partner is taking potentially dangerous drugs, its well within reason that she does not want that kind of risk of possible side effects that could cause serious health or intimacy issues down the road.
Basically its not that she doesnt understand or care for you, its that she does care for you so much, that the thought of possibly losing, or otherwise altering, the person she loves is unacceptable.
Now is it as dangerous as everyone makes it out to be, no. But you would be kidding yourself to not accept that there are very real risks involved. And if you are with someone for the long run, that's a risk they are taking with you.
My advice would basically break down into two parts:
1. If you do not see this this relationship going the distance, and you have absolutely no intentions of marrying her, or otherwise long-term commitment. Then you don't have to tell her. If she finds out, or if something god forbid goes wrong, then you already knew it wasn't going to last anyway.
2. If you really see a long term possibility here. You owe it to her to let her know. As stated earlier, its a risk she is taking to, and she has to be comfortable and be accepting of the risks involved that could effect her life, and meaningful relationship. Doing anything else borders on shady and deceitful, and is unfair to her in every way.