I would speak with your confessor first. Then your doctor. Just to be safe on both ends. :)
I am often forced to fast. I offer it up (as usually I am in a lot of pain those days too) accordingly. If greater merit for those souls in greater need is granted because the day is a Friday (or any of the rogation days) God be thanked.
If you were to choose another day other than Friday, my choice would be Wednesday, as this follows the practice of Traditional religious communities.

I will take any physical/medical benefits that come with fasting, as a secondary to the spiritual benefits.