This often happens if the bipolar person is in a depressive phase.
Sometimes nothing happens- good or bad. Sometimes the antidepressant can trigger a manic cycle. Close communication with doctor is imperitive in the diagnosis, treatment and follow up of a complex and chronic problem like bipolar. Nimh has a great web site: www.Nimh.Nih.Gov/health/publications/bipolar-disoreder/index.Shtml.