Mirtazapine is a heavily sedating antidepressant that comes in three doses: 15, 30, and 45 mg.
If you are prescribed this a physician will usually start on 15 mg/day for a week then 30 mg/day. Mirtazapine acts more like a anti-histamine at lower doses in that it knocks you out for at least 10/12 hours and leaves a residual grogginess for most of the next day. The first time I took this is 2007 I had to stop taking it after a couple of weeks because of this. Although I started on it again in 2012 and stuck through the first 2/3 weeks wherein it becomes more tolerable. I found the 15 mg much more sedating then the 30 mg, but the 45 mg dose more sedating then both put together. Beyond sedation I have found mirtazapine to be very tolerable with the only other side effect being increased appetite. I always take it in the evening along with olanzapine.