They can spur a manic or mixed episode. If an antidepressant is used it usually is only used along with a mood stabilizer, and the patient needs to be very alert to changes in mood, and the doctor needs to be aware and watching for signs or symptoms. For some it is that they have history of this happening and so the doctors will be reluctant to use an antidepressant again. It depends on how bad you need the treatment, is the risk worth it, and how bad was the manic or mixed episode you had in the past with the use of antidepressants. Some people are fine as long as they have a mood stabilizer too. Some people, it is a big no-no.
Some of the time a mood stabilizer is all that is needed to get both manic and depressive episodes under control. Some mood stabilizers, Lamictal for one, are supposed to be specifically good for depression in bipolar.
I cannot use antidepressants. They have put me into dangerous mixed states in the past. I exercise, journal, do yoga once a week, therapy once a week, and try to remind myself that the messages I am receiving--those negative, pessimistic tapes that play over and over, I remind myself that they are not what is real. They are depression trying to deceive and delude me, don't listen to its lies! Then I try to keep busy, hold on tight until it is over. I don't deny myself my feelings-- I cry when I feel the need to cry and swear when I get frustrated. I need that release. So, sometimes all of this works well, sometimes just a little bit. Better than nothing.