How can amitriptyline to cause your metabolic rate to slow?

It Does Not: It just causes increased appetite. That is the cause of weight gain. (it can also cause constipation which would cause a bloated abdomen and weight gain.).

Not quite right: I don't know that amptriptylene actually slows your metabolic rate, but it can increase appetite leading to weight gain. Also it has effects that slow your GI (gastrointestinal) system down, making constipation possible unless you're careful to drink fluids & eat good leafy greens, other vegetables, and fruits. This is different from the metabolic rate in your body, though.