Common mistakes that make Plan B less effective and what you can do about it

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This article was medically reviewed by Lauren Demosthenes, MD, OB-GYN, senior medical director at Babyscripts.

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Our stories are reviewed by medical professionals to ensure you get the most accurate and useful information about your health and wellness. For more information, visit our medical review board.

• Plan B is 95% effective if you take it within 24 hours and 89% effective within 72 hours.
• But it might be less effective if you are ovulating, or have unprotected sex after taking Plan B.
• It is also less effective for women over 155 pounds — if this is the case, try a pill like Ella.

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Plan B has a 95% chance of preventing pregnancy if you take it within 24 hours. If you take it within 72 hours, your chances reduce to 89%.

Note: Plan B, sometimes referred to as the "morning-after pill," is an over-the-counter form of emergency contraception you can take to prevent pregnancy after having unprotected sex.

"Plan B is FDA approved for use up to 72 hours after unprotected sex. It can be used up to 120 hours after, but the further out from intercourse, the less effective it will become," Nicole M. Bender, MD, an OB-GYN at Keck School of Medicine of USC.

So, for max protection, take the pill as quickly as you can get it.

How Plan B works

The active ingredient in Plan B that prevents pregnancy is a hormone called levonorgestrel, which is a synthetic version of progesterone.

Aileen Gariepy, MD, OB-GYN at Yale Medicine and associate professor at Yale School of Medicine says that levonorgestrel works to prevent pregnancy by:

• Delaying or stopping ovulation, which is when an egg gets released from the ovary.
• Preventing fertilization by slowing down the path of the egg or sperm so that they don't meet.
• Stopping a fertilized egg from implanting in the uterus by thinning the uterine lining.

Plan B won't work if a pregnancy has already been established, says Gariepy. The only way to know for sure if you're pregnant is to take a pregnancy test. However, you can also look out for early pregnancy symptoms like a missed period, nausea, vomiting, or sore breasts.

Note: Taking more than one dose of Plan B after an act of unprotected sex will not make it more effective, Gariepy says.

Additionally, if you've ovulated recently, some research indicates that Plan B does not work as well.

Ovulation typically occurs right in the middle of your menstrual cycle, such as on day 14 if you have a 28 day cycle. You might also notice clear and stretchy vaginal discharge. To be extra sure, you can purchase over-the-counter ovulation strips to determine if you're ovulating.

Note: Plan B is different from an abortion pill, and it will not terminate an existing pregnancy.

How long is Plan B effective

Plan B starts working soon after taking the pill. Peak levels of Plan B in your bloodstream will occur about 1.7 hours after taking the pill.

Depending on where you are in your cycle, Plan B will likely delay ovulation for a few days, says Gariepy. And the levonorgestrel should leave your system after about a week or so, depending on how fast your metabolism is, Gariepy says.

However, it's important to note that plan B only works for the episode of unprotected sex you have already had. If you have unprotected sex shortly after taking plan B — like later that day or the following day — then you will need to take another dose of plan B to prevent pregnancy.

Side effects of Plan B

Plan B may cause side effects, which Gariepy says are most likely to occur within a few hours of taking the pill, since the peak levels in your blood will be around 1.7 hours after. Side effects will usually resolve within 24 hours and include:

• Nausea
• Vomiting
• Headache
• Dizziness
• Breast tenderness
• Vaginal bleeding

4 things that make Plan B less effective

Be aware that there is a risk Plan B might not work for you if you fall into one of the following groups.

1. People who throw up shortly after taking the pill: You need about 2 hours to absorb the levonorgestrel, so if you throw up within two hours after taking plan B, take another dose to make sure it enters your system.
2. People who are already ovulating: If you have already ovulated (an egg has already been released), Plan B may not be as effective in preventing pregnancy, Gariepy says.
3. People who have unprotected sex right after taking Plan B: "If Plan B delays ovulation by a few days and then you have another act of unprotected sex a few days later, you can still get pregnant," Bender says. So, if you want to be safe, be sure to use protection the next time you have sex.
4. People above a certain weight: If you have a body mass index (BMI) of 26 or higher, Plan B may be less effective, says Bender. A 2015 study found that as someone's body weight went up, the effectiveness of levonorgestrel went down. In fact, the researchers reported that women who weighed 187 pounds, or more, were equally at risk of getting pregnant after unprotected sex whether they used plan B, or not. A prescription emergency contraception called Ella, is more effective in women who weigh 155 pounds or more. Ella can be used up to 120 hours after unprotected sex, though it's more effective the sooner you take it.

Note: A prescription emergency contraception called Ella, is more effective in women who weigh 155 pounds or more. Ella can be used up to 120 hours after unprotected sex, though it's more effective the sooner you take it.

Medicines you should not take with plan B

Some medications may interact with Plan B, making it less effective, since they can decrease the amount of levonorgestrel in the blood. These medications include:

• Barbiturates
• St. John's wort
• Oxcarbazepine
• Some HIV/AIDs medications

For a full list of medications, visit Plan B One-step's website here.

Insider's takeaway

Plan B is a type of emergency contraception that is very effective at preventing pregnancy if you take it within 24 hours of unprotected sex. The longer you wait to take it, the less effective it becomes.

Plan B is not a substitute for birth control. It's meant for emergencies, not to be taken regularly. "Plan B is a great back-up plan — meaning it's not meant to be someone's first line of defense against pregnancy," says Gariepy.

Ashley Laderer is a freelance writer from New York who specializes in health and wellness. Follow her on Twitter @ashladerer