On 112.5mg effexor (venlafaxine) for 6mths. mood dropped unexp. so upped dose to 150mg. 1mth later mood worse so now on 30mg mirtazapine. what caused mood drop?

Wrong medication: Ask your treating physician about using Zoloft, Lexapro, (escitalopram) or Celexa- certainly more rapid acting: I would also advise psychotherapy with a behavioral counselor ( therpist) using CBT ( cognitive behavioral therapy ).

Could be many causes: There can be a number of triggers to low mood, including: medical illness, stress, loss, relapse of depression, seasonal changes and even just normal variation. It would be best to talk to your doctor about the possible explanations to see if you need any further medical work-up or if adding other treatment components could be helpful (e.g. psychotherapy).