I have been taking Prozac 20 mg for about a month. I had some nausea in the beginning, but no worse than I already experienced with IBS. I also found that I was not sleeping as long as usual, waking up an hour or two early. The biggest change I have noticed is less abdominal pain. It is not completely gone but much better. And even though I would still not consider my BM's normal, I have not had any constipation.