Will taking birth control instantly stop your period if you are already on it?

generally not. Birth Control takes about a month to work. certain kinds are made to stop your period but others contain a week of blank pills (pills without a hormone) to allow your period to happen once a month, even while your birth control is in effect. for some people, like me, it stops your period anyway though. sometimes you have to switch different brands to see which one is right for you.