It is a synthetic chemical, and long term use might impact a woman's cycles. Women do get pregnant on the Birth Control Pill. Women have irregular cycles on the Birth Control Pill. Each woman is unique, and the impact of a particular method on a women are unique. An example of this is a tubal ligation.
Some women have a tubal and have no problems from it.
Sometimes problems develop after the tubal, but can't be directly connected to the tubal. Of course a tubal is to be considered permanent sterilization, but sometimes women want to reverse them. This becomes a costly and anxiety producing situation. The Depo Provera injection can be a wonderful method for some women. For others, they notice depression, or excessive bleeding, or more moodiness.