I've read that as well. That obesity can affect how well contraceptives work due to absorption. I'd say that yes, you'd be wise to use a back up method of birth control along with it if pregnancy is out of the question. You had unprotected sex how long ago? We'll start there. But usually you can get an accurate pregnancy test at 2.5 weeks after the sexual encounter. If it has been that long and you haven't gotten your period, you can take a test.