Isint birth control bad for you?

ok you know some of those birth controls that say oh you just get 3 periods a year or you get it every 3 months or whatever, isint that bad? women are supposed to get their periods every month to maintain healthy bones, this anorexic girl didnt get her period for 1 year and her bones are now like a woman 20 years older than her, isn't it dangeruos to take them?