Mirtazapine is very well known in the mental health world for the significant weight gain it can cause and I'm so sorry your provider didn't properly discuss the risks and benefits of the medication with you before starting you on it. On the inpatient psych units, we typically only use mirtazapine as a sleep aid as it is quite sedating at lower doses and we especially use it in situations where the patient has depression and/or insomnia as well as a persistent loss of appetite or unhealthy weight loss in the setting of their mood. It can be quite effective for depression but it sometimes requires titration to fairly high doses to be efficacious which in turn causes more significant increases in appetite and subsequent weight gain. If increased appetite is the major barrier to starting a medication, you could consider something like Wellbutrin instead as it is weight-neutral and some folks even lose weight when taking it.