I have talked to my pdoc about being very concerned about weight gain from meds. I have had a terrible reaction to almost all meds I have taken to treat anxiety/depression. I have avoided trying a few well known for causing weight gain, and I have also tried some that I knew might cause it (because chances are I would have another side effect first and not be able to tolerate it anyway...which was indeed the case). I do not have an ED, but I have, as an ex-T once described it, "a complicated relationship with food." I am very sensitive about what food I eat, how much I eat, where I eat, and who I eat with...I don't need a weight gaining drug in the mix.
I also encourage you (as others have here) to talk to your pdoc and make sure that you are put on a weight neural med, as well as talking to your T about it to make sure you get the support you need.