Mirtazapine can cause rapid weight gain. It did for me the first time I took it when it was a new med in the 90s. When I took it again a few years later I was reluctant because of that but the pdoc increased the dose and I didn't gain at all. For some reason the higher the dose the less the appetite increase/weight gain and the less the sedation (which is a downside if you want sedation). I seem to remember that I was still able to get to sleep on it though at the higher dose. At 45-60 mg it was pretty much side effect free.