the hormones in them have different effect on different people.... you should discuss any changes they cause with your doctor. (If they give you weird side effects.. I would consider trying other methods)
It made mine stop but at the same time it made me loco (more so than usual!)
It depends on the person- some women have long heavy cycles and others don't have any cycles.
Because your messing with nature and there are side effects.
your changing the natural balance in your body with hormones , so your periods are going to change in length, occurrence , and severity