Verified answer

If you had sex, took plan B, then had sex again, it might not be effective. If you had sex twice and then took it (or are considering taking it), it would be effective.

Plan B is a hormonal medication that, most often, prevents the egg from being penetrated by the sperm. If the egg is already fertilized, it will prevent the egg from attaching to the wall of the uterus.

However, Plan B is only effective for about 5 days after the intercourse (in this case, the first time) and is most effective if taken within 72 hours. So, if you haven't taken it yet and are wondering if you should, the best thing to do would be take it as soon as you can.

Also, if you aren't 100% positive that you and your boyfriend are both STD-free, you should consider getting tested (both of you) to make sure that you are safe having unprotected sex.

Plan B is helpful in preventing pregnancy 24-72 hours after intercourse, with decreasing effectiveness over that interval. It doesn't matter if you have sex more than once, but what does matter is how long ago the first unprotected sex was.