Megace...and UNWANTED weight gain

I have been subscribed Megace to help with "bad" cells found in my uterine lining after biopsy. I have been reading about it's use and side effects...I am freaking out....it is used to make people GAIN weight....I have forever struggled with a sluggish metabolism, and have always had trouble losing weight....I am already 20 lbs overweight! What I want to know is...is it simply helpful in patients who need to gain weight because it stimulates appetite...or even if you don't eat it somehow makes you gain weight? I can ignore hunger pangs...but if it is a hormonal action or some other that causes the weight gain...I need to talk to a dr. regarding an alternative if there is one. THanks in advance for any advice! Jennifer