How will the birth control shot affect your menstruation cycle?

More and Less: Sometimes the birth control shot, the depot Provera (medroxyprogesterone) shot, causes a period to become heavier for awhile, and in some women it causes the period to become lighter. Of course some women's period doesn't change at all. Confusing, i know. Some will note some breakthrough bleeding, the majority, however, note lessening or even absence of a period on this shot.