As best I understand it, Mojoforyeshua, if your fiance’s need for medical treatment for her condition that has a contraceptive quality as a side effect then that treatment (with its side effect) is ok.
However, it also seems to me that if such treatment were not available or required, it would be unwise to marry and allow oneself to get into a situation where intercourse resulted in a pregnancy that represented a significant danger to your wife’s life, and indeed the child’s. It would be better to remain unmarried and chaste (since marriage is generally understood to be ordered towards the possibility of procreation) since chastity is the only sure-fire method of preventing a pregnancy. Contraception does, after all, fail from time to time and then what would you do if your wife got pregnant? You might be sorely tempted to abort the child, which would be as wrong as wrong can be.
As someone with digestive issues of my own that has resulted in frequent and prolonged nausea, I have nothing but the deepest sympathy with your fiance. The situation must be almost intolerable for her. But I think that you need to step back and recognise the seriousness of the situation and what the only practical and reliable answers are, as difficult and painful as that will surely be. I wouldn’t be in your shoes for a moment, I can tell you that. I really do see the pain this will be causing you.