I really wouldn't recommend anyone going onto Mirtazapine unless either you're in an institution where you don't have the capacity to make the decision or you have tried EVERYTHING else.
For people who Mirtazapine has worked for, I am really glad for you.
For me, I had exactly the same side effects from the first week of the medication to the seventh week, with none of the benefits. I used to take 30mg at night, and would wake up extremely weak and without the energy to actually get out of bed (\"Hungover\" is the best way to describe it).
I was getting even less done during the day (like having a wash, tidying up, cooking) then I did before I went on it, and my suicidal thoughts increased dramatically. I would also snap really aggressively for no reason whatsover. Considering I was switched to this by my local Crisis Team, it's done nothing but made the crisis worse!
*HOWEVER* Yesterday I was diagnosed with Bipolar, and was switched to another anti-depressant along with a mood stabiliser. I already feel fantastic just not being on Mirtazapine anymore, although the side effects in my case might have continued because the medication was causing me to \"Rapid Cycle\" from elated to depressed, quite often a few times in the same day. If you are given it as a medication for bipolar, maker sure they put you on a mood stabiliser too.
But whatever the case, if you are just about to start it, prepare to be completely knocked out, possibly for the whole duration on it! And I would recommend trying to ask your doctor if there are any alternatives!