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Well, the answer to your first question is that , YES , unipolar depressive disorder may end up after a while becoming a bipolar disorder , due to occurrence of either a manic episode or a hypomanic episode . This kind of transition from a unipolar disorder to a bipolar disorder is sometimes seen , so this is not something unique ....
Answer to your second question is that , it is not likely that venlafaxine alone might have caused mania , well any anti depressant , be it citalopram or venlafaxine or any other anti depressant drug for that matter can unmask mania in a depressed person , but there is condition attached and that is mostly such unmasking of mania occurs only in those people who are genetically vulnerable to it , so i do not think venlafaxine alone could have caused this unmasking of mania , it could be contributions from all the anti depressants that you have used so far , that might have caused this manic episode thereby causing the transition to bipolar disorder from unipolar disorder, but this mania could also have been caused because you are simple vulnerable to it , either due to genetic preponderance or unknown reasons..
So, now what i suggest is that , you ask your doctor to add 1 more mood stabilizer drug while continuing with lithium and the drug's name is XXXXX XXXXX this will ensure that your mood swings and other bipolar disorder symptoms are kept in check..
I hope this helps..
Wish you all the best..
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