The birth control pill is the most popular reversible contraceptive of sexually active women trying to prevent pregnancy, according to the Centers for Disease Control and Prevention. And why not? It’s easy to use, has few side effects and it works -- as long as you take it at the same time each day. The Pill works in part by suppressing ovulation, so each time you miss a pill you increase the odds that ovulation might occur. Certain medications, including some antibiotics, anticonvulsants, HIV medications and the herb St. John’s wort, also may weaken the pill’s effectiveness. Next slide: What you need to do to make sure it's working for you.