I am on 100mg Zoloft (sertraline) and have gained a lot of weight over several years. Is this a side effect?

Side effects. Weight gain is a potential side effect for any psychotropic medication. Some agents more pronounced than others. Dietary attentiveness, exercise and review of all meds. Used may decrease this issue. Careful balance is prudent between risks & benefits, and should be reviewed periodically during the course of care.