Antidepressant drugs usually do not cause the problem of weight gain, if they have caused weight gain, it may be due to the following three reasons: 1. All antidepressant drugs can effectively improve mood, which makes you feel better and increase your appetite. At this time, if you eat more, and at the same time exercise less, there is a possibility of weight gain during the medication. 2, it is possible that some antidepressants have certain sedative effects at the same time, such as mirtazapine or sertraline, trazodone, etc.. These drugs may have the problem of weight gain, because drugs with a certain sedative effect, especially mirtazapine and the like, the sedative effect is more obvious, very likely to appear lazy, reluctant to move, there may be weight gain or even fat. 3, it is possible to take antidepressants booster, such as olanzapine, risperidone, etc., these drugs may cause weight gain. Olanzapine, in particular, has a higher incidence of weight gain as a side effect. The above three reasons may lead to weight gain from taking depression medication.