How fast and affective do birth control pills work in preventing pregnancy? I've been using it for 16 days now and had sex on day 11 and with other birth control methods but I'm still a little worried? I guess I'm worried because I haven't been taking it that long and if I wasn't using any birth control it would've been a very fertile time for me and I had all the symptoms that the pills were working but after having sex they all went away and now all I feel is cramps and I don't remember reading that cramps were a side affect to it. And it's kind of hard trusting a little pill.