Because there are different kinds at different dosages, not every type works the same for everyone. They can also have some serious side effects. (I had a friend who had a stroke caused by her birth control pills.) It also helps regulate people taking them incorrectly. They effect the hormones in your body which if used incorrectly (like someone who skips the week of the placebos) can really mess your body up. They are not something to be dealt with lightly.