I think it is the depo provera shot that prevents periods for three months. You only get the shot once every three months. Everything has side effects. I don't know that the side effects from depo would be any more serious than with other birth control, but it is a little scary that once you get the shot, it is in your system for 3 months, with no real way to discontinue it until it naturally is eliminated from your system.

More information:
http://www.birth-control-comparison.info/bcdepo.htm