Plan B is a contraceptive pill that can prevent pregnancy after unprotected sex, or after another birth control method failed. Mistakes happen — that’s why emergency contraception is here for you. However, the effectiveness of Plan B depends on when it’s taken. ‍

How does Plan B work?‍

Plan B acts primarily by stopping the release of an egg from the ovary, or, ovulation. Therefore, even if there is sperm present, there will be no egg to fertilize. If fertilization does occur, Plan B may prevent the fertilized egg from attaching to the womb. (1, 6). ‍

When to take Plan B‍

A “morning-after” pill — also known as Plan B One-Step, Take Action, Aftera, EContra, or After Pill — works best when taken between one and 72 hours after unprotected sex (1, 2) . For maximum results, head to your local drugstore and pick up emergency contraceptive immediately after unprotected sex, or as early as possible, if you want to prevent pregnancy or if your primary birth control method failed. For example, if the condom broke, you did not use the pull out method correctly, you are not on the birth control pill, or something else.‍

How effective is Plan B if taken at the right time? 

Plan B is up to 98% effective for preventing pregnancy if taken within 24 hours of unprotected sex. Efficacy drops to 89% if taken within 72 hours. ‍

When is it too late to take Plan B?‍

According to The American College of Obstetricians and Gynecologists (ACOG), Plan B has decreasing efficacy with time after unprotected sex and is labeled for use up to 72 hours. It is only moderately effective when taken up to 5 days after sexual intercourse (2). 

“There is some limited efficacy at 96 hours, and by day 5 you are 6 times more likely to get pregnant than on day 1,” Andrea Martin, DNP, CRNP, WHNP tells O.school. “Taking Plan B after 72 hours won’t hurt you, but it most likely won’t help prevent pregnancy either. At that point you should consider an IUD or Ella (ullipristal) to prevent pregnancy.”‍

Who should consider Plan B‍

If you forgot to take a birth control pill, found a hole in the condom after sex, or made an error when using the rhythm method or the pull out method, you stand the chance of getting pregnant. Anyone who is in this position and isn’t ready to expand their family is a good candidate for Plan B (2).

Even if you are on your period at the time of the mistake, or another point in your cycle when chance of pregnancy is low, practicing caution by taking an emergency contraceptive might be a smart idea. Or, at the least, it will give you peace of mind.

“In a standard 28-day cycle, a person ovulates on day 14 and is most fertile days 8-19, and less fertile the other days,” Martin says. “However, every person is different and it can take months to truly understand the patterns of an individual cycle. I encourage everyone to get familiar with their cycle and their patterns, but if you don’t want to be pregnant, it is better to be safe than sorry and take emergency contraception.”‍

Who should not use Plan B‍

Anyone who is allergic to the ingredients in Plan B should not take Plan B, Martin asserts. Pregnant people should also avoid taking emergency contraceptives. “Not because it would harm the pregnancy,” says Martin, “but because it won’t work” (3).

Certain prescription medications may interact with Plan B and make it less effective. (5) If you are on the antibiotic Rifampin, the antifungal Griseofulvin, or HIV or anti-seizure medications, speak with your doctor to determine whether Plan B is right for you.

Your weight can impact the effectiveness of Plan B

If you take an emergency contraceptive like Plan B or one of its generic counterparts (see: Take Action, My Way), and you weigh more than 165 pounds, it may not work. In this case, Ella, another emergency contraceptive option, might work better for you. If you weigh over 195 pounds, pills may not work for you at all. Fortunately, no matter your weight, having an IUD inserted will act as an effective contraceptive within 120 hours of having unprotected sex (2) .

Plan B alternatives 

You can get Plan B over the counter at your local drugstore, though there is a cost barrier to obtaining Plan B of about $45-$50 dollars. However, the effective ingredient in the drug is 1.5mg of Levonorgestrel (a type of Progestin), and taking a generic version made of the same medication and dosage is just as effective. This means Take Action, My Way, Option 2, or another generic might be a more accessible choice for you. If cost is a concern, call your nearest Planned Parenthood, your local health department, or another family planning clinic to see if they can help you get emergency contraception free or at a lower cost. 

If you have passed the 72 hour window for an emergency contraceptive and have decided you don’t want to risk a pregnancy, IUDs present a great alternative (1, 2).

“Paragard, Liletta, and Mirena can be used within five days of unprotected sex and are 99% effective,” Martin tells O.school. “They are just as effective on day one as they are on day five and go on to provide years worth of protection.” 

Another alternative on the market is Ella (ulipristal acetate). This can be used up to 5 days after unprotected sex, and is more effective than Plan B. You’ll still want to take it as soon as possible if this is an option (1, 2, 5). 

‍How many times can you take Plan B? ‍

There is no limit to how many times you can take Plan B. But, if you have unprotected sex, take Plan B, and then have unprotected sex again, it is important to take another dose (2). 

“Note that while it is safe to take Plan B multiple times, it isn’t a reliable form of long-term contraception and there are a plethora of more effective birth control options to consider,” says Martin.

‍The bottom line‍

Whether you opt for Plan B or a generic version, the morning-after pill is a highly effective backup when your first line of defense fails. If you’re outside of the recommended window, exploring an IUD is not only a valid emergency contraceptive, but a safe way to ensure you don’t end up needing a “Plan B” for years to come.