When used perfectly, the combined pill (which has estrogen and progesterone) and the progestin-only minipill are 99.7 percent effective. But that number drops to 91 percent with so-called "typical use"-meaning the way that most women take them. "In some cases,the failure rate can be as high as 20 percent because they forget to take it regularly or they run out of pills and don't get a refill right away," notes Andrew M. Kaunitz, M.D., associate chairman of obstetrics and gynecology at the University of Florida College of Medicine-Jacksonville.