I wouldn't be surprised that Mirtazapine affects balance somewhat - I take it myself, and it is known to be more sedative than many other anti-depressants. Sedated generally means slower reaction times and dulled physical perceptions - even if your dose is too low to notice these things consciously, it could easily still affect the fine motor skills needed to balance like that. If you have no other bad side effects, I doubt it is very much to worry about - but all the same, mention it to your doctor next time you need a prescription just to be sure.
I'm not even going to try the test as I have slightly wonky proprioception, and have always had hopeless balance - I would struggle enough with a field sobriety test most days!
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