Educate her on it - this is the best method, as I have found from years of personal experience. My wife (along with everyone else not involved in medical research or the bodybuilding industry) believed that steroids referred to only one product or pill and that they would send you into a murderous rage before killing you do to liver and kidney failure all over night! People don't even understand 'steroids' refers to an entire collection of anabolics, and many of them have this misconception that you can take steroids and wake up looking like a magazine cover of FLEX, WITHOUT lifting weights!
Anyway... long story short, my wife went from an uneducated steroid fearing media infected propagandized individual, to someone who regularly tries to find room in the budget to purchase another cycle for me, and is always supporting my quest to become/remain very muscular.
Initially, once any audience, your girlfriend in this situation, becomes aware that steroids are no more than a sub set of pharmaceuticals used to improve health, muscle wasting conditions, expedite healing and recovery, over come and cure hormone deficiencies which leads to greatly improved quality of life, and so on and so forth with an almost limitless assortment of benefits in the medical community, she/they will begin to become receptive. They are given to aids patients and children burn victims, all the way to aging men and women who wish to regain their once enjoyed vitality and value of life... all of this merely translates into being an effective collective of drugs, like anything else, that has been employed by learned athletes who wish to recover faster and be more productive in their ambition. The tragedy isn't the fact thousands/millions of men (and women) are using AAS, but due to the heavily politicized and skewed laws put into effect by an ignorant and largely unconcerned congress who cast their votes as a mere "look what I did" gesture to get pats on the back from their constituents, grown law abiding and well versed experienced athletes are forced to seek out these products often resulting in under ground lab patronage with no quality or safety measures put into place.
There is an unstoppable inner desire that is a crucial component of the human condition that drives us with immeasurably intensity to achieve and succeed - be the merit of our own definition while engaged in our own individual pursuits. The US Government cast the science of safety and efficacy aside, and outlawed these substances, forever perpetuating a horrid and unrecoverable bad reputation affixed to the taboo subject of steroids.
You girlfriend will come around when she realized you are passionate about your endeavor, and have invested years into the research and are personally convinced and sincerely attest to her that your health and demeanor will not be eroded in light of your calculated and cautionary controlled usage of these additives.
anyway as I've already explained I have chosen the selfish route because I know ehats best and I'm rational and she's not 
