I came off Citalopram as it was not helping and went straight onto Mirtazapine.
The side effects started immediatley :

Very agitated/short tempered and easily annoyed becoming very angry at the least little thing.
Upset stomach
Bad headaches
Excessive sleeping
Unable to keep still – arms can’t stop moving
Slurred speech
Inability to concentrate on anything – can’t read/watch TV
Severe indigestion.

This got gradually worse until after a week the headaches were completely debilitating and the indigenstion had got so bad I felt as though I had swallowed a house brick that had stuck in my throat and wouldn't go up or down. the pain spread up to my inner ears it got so bad.
The constant headache left me barely able to see and unable to stand light
No amount of pain killers or indegestion tablets made any difference at all.

I have decidd to stop taking the tablets completely (against all advice) because I just cannot stand the pain any longer.
I will try to get an appointment with my doctor on monday.