I was on Effexor for a few years, it worked great for the most part. HOWEVER, without warning it decided to poop-out on me and caused me worse problems then before I even started on the medication. I only recently came off of it, and have felt much better since doing that.
Why some of these meds work for some and poop-out on others, they don't know. You could go on it and have complete success, and never have the problems that I did. Like I said, when it was working I did great on it!