How Does It Prevent Conception? Emergency Contraceptives put the brakes on ovulation or keep the sperm and egg from joining, depending of course on where you are in your cycle. The Morning After The Morning After Pill? EC works best when taken within 24 hours, try having some stocked so you don't have to make a stressful dash to the drug store. You can take it up to 72 hours after a lack of or failed contraceptives. Of course the longer you wait the more likely you will be to get pregnant. Side note: When used correctly Plan B will prevent 7 out of 8 unplanned pregnancies. What Are The Side Effects? Most women don't experience side effects, but when they do they're usually mild and limited to bloating, spotting, cramping and nausea. How Will I Know If It Worked? You will probably get your period on time, but don't be surprised if Plan B throws it off schedule a few days, this is normal. However, makers advise taking a pregnancy test if you're a week or more late. For more on Plan B visit the website.