I am on ami and have not put weight on, are you sure you are just not eating more and not exercising enough. Gabapentin will also put weight on, and be warned can be hell to get off of. Noratriptyline I am sure is not expensive, you can look up how much all these pills cost, ami is the cheapest, I then thought Nora, then it goes up and pregagablin is the most expensive.
It could also be because it has stopped your IBS you are getting more nourishment from your food.
I take it for nerve pain, but it has also stopped my IBS and constant back ground headaches. Try the Gaba but I know a lot of people struggle with the side effects of it, they all make you drowsy for the first few weeks but I have no side effects at all now.
Gaba is used for people who have seizures so works very differently on the brain, my GP offered me all of the different types and I decided on Ami.