You may not gain too much weight on mirtazapine, as most studies have found that it does not cause a lot of weight gain, and most of the weight gain happens at the end of about 4 weeks. There was a study done by Thase and colleagues about the weight gain effects of Mirtazapine, and they found that the incidence of self-reported weight gain after 12 weeks of mirtazapine was 21%.