The hormone in Plan B, levonorgestrel, keeps the ovaries from releasing an egg. Plan B is not percent effective, so a pregnancy after taking Plan B may occur. Planned Parenthood explains that women can take Plan B up to hours after sexual intercourse. But the sooner a woman takes Plan B, the more effective the drug is. The McKinley Health Center at the University of Illinois notes that Plan B is 95 percent effective when taken within 24 hours; the effectiveness drops to 61 percent if taken 48 to 72 hours after intercourse.