If I remember correctly, it is supposed to be 99% effective. That sounds great, till you end up in the 1%, like I did. Make sure if you are taking antibiotics for anything that you use another method as well, because antibiotics wipe out the effects of bcp. In addition to a kid (who I love dearly, but hadn't bargained for initially), I ended up with a blood clot in my lung because of the pill. That's also in the 1% category. I'm just lucky that way. But 1% of the millions who take the pill is a LOT of women!