I went on two drugs which had read about the side effects beforehand and knew weight gain was a big one, Mirtazapine to help me sleep better (changed from citalopram) and Pregabalin (pain med).
I have put on so much weight, a good few stone and I hate it. I'm not even eating more, less in fact including snacking on fruit if anything (grapes usually) and having the lowest calorie sandwich I can find for lunch, often leaving the crusts (or whole sandwich) and then a 10 calorie jelly and low fat yoghurt, not a big breakfast or tea either. But the weight has just piled on and won't shift, I'm not very mobile though which doesn't help.
You have to look at whether the benefits outweigh the side effects though it's now always easy. I've been struggling to cope a lot recently but am so loathe to ask increasing any of my meds in case my weight goes up even more.