She should be more afeared of you gettin' so swolled up that you trade her in for a better looking, and newer model of chick.
That, or she might have to get used to sharing you, since not only will you be so fucking buff, you will also behave like a rutting pig in heat.
Cancer? What are you, a lab rat?
Anyways, real problems associated with what you are contemplating: abscesses, high bp, gyno, loss of hair.
Well, for the rest, you can do some searches. Also, why and how to avoid most/all of these sides, you can do a search for that too.
My advice to you however, is to not tell your chick. Why? What is the upside vs. downside? Well, the upside is she will say 'ok'. Big fucking whoop. The downside is she might tell all your buddies, and her g/f when/if y'all ever break up, or even get into a fight. There was a thread a few weeks ago about some dude who told his g/f, and he said she seemed totally ok with it. However, when they broke up, she told everybody, including his boss.
Think about it. And be careful, chicks are fucking insane.