How Effective Is Plan B Within 12 Hours

How Effective Is Plan B Within 12 Hours

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

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.

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

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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.
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:
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.
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.
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 :
Be aware that there is a risk Plan B might not work for you if you fall into one of the following groups.
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 :
For a full list of medications, visit Plan B One-step's website here .
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.

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How effective is Plan B and how late can you take it?

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Medically reviewed by Sally Chao, MD . Last updated on July 30, 2021.
Plan B ( levonorgestrel ) is estimated to be 60% to 94% effective at preventing pregnancy when taken within 3 days (72 hours) after unprotected sex. Somewhere between 0.6% to 2.6% of women who take Plan B within this time frame will still become pregnant.
While Plan B can be taken up to 72 hours after unprotected sex, its efficacy decreases with time, and it is best to take it as soon as possible. For example, the chances of pregnancy are higher if Plan B is taken 4 days after unprotected sex, but are subsequently lowered if taken 3 days, 2 days, 1 day or within 24 hours after unprotected sex. However, the American College of Obstetricians and Gynecologists notes that some studies suggest that it can be moderately effective at preventing pregnancy up to 5 days after unprotected sex. Five days after unprotected sex is the latest time with which Plan B has been shown to lower your chances of pregnancy.
The efficacy of Plan B is also affected by the time at which it is taken within your menstrual or reproductive cycle. Plan B works best when taken before ovulation, or when an ovary releases an egg. Once there is a surge in the hormone that initiates ovulation and ovulation begins, Plan B cannot prevent pregnancy. Its primary mechanism of action is to change hormone levels such that ovulation is delayed. This is why it is important to take Plan B as soon as possible after unprotected sex. As treatment is delayed, ovulation becomes more imminent (if it has not yet occurred).
A review of studies in Clinical Obstetrics and Gynecology showed that among 492 women who took Plan B after unprotected sex, those who took it on or after the day of ovulation had the same pregnancy rate as would be expected among women who used no contraception at all. By comparison, none of the women who took Plan B before ovulation became pregnant.
In the event of another unprotected sexual encounter after Plan B is taken, Plan B will not protect against pregnancy—you will need to take Plan B again after another unprotected sexual encounter.
Plan B may also be less effective at preventing pregnancy in women who are overweight or obese. Studies reviewed by Cochrane suggest that the dose of levonorgestrel in Plan B has a lowered potency when taken by women who have a higher body mass index.
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Drugs.com provides accurate and independent information on more than 24,000 prescription drugs, over-the-counter medicines and natural products. This material is provided for educational purposes only and is not intended for medical advice, diagnosis or treatment. Data sources include IBM Watson Micromedex (updated 5 July 2022), Cerner Multum™ (updated 27 June 2022), ASHP (updated 16 May 2022) and others.
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How Effective Is Plan B Within 12 Hours

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