How Long After Sex Can I Take Plan B

How Long After Sex Can I Take Plan B
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Published: Mar 29, 2018
Last Updated: Oct 26, 2019

Answered By: Dr. Brian Staiger Pharm.D

I take combination pills and I took the second pill of the first week of a new pack 9 hours late. Do I need emergency contraception or any back up methods?

I started my combination birth control pill pack a day earlier, I was supposed to take it on Sunday but I started Saturday. I don’t want to change my usual start date...I want to continue starting my...

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In our latest question and answer, the pharmacist discusses the efficacy of using Plan B 4 hours after sex.
My girlfriend and I had sex towards the end of her period and took plan b 4 hours later, could she be pregnant still?
Plan B (levonorgestrel) is a progestin containing emergency contraceptive used to prevent an unintended pregnancy after unprotected sex or in situations where the contraceptive method used fails (e.g. a broken condom).
Plan B is most effective when used within 72 hours of unprotected sex. In your case, taking Plan B 4 hours after intercourse is within the time frame in which Plan B is thought to be most effective.
Plan B (levonorgestrel) is thought to work via multiple mechanisms to prevent pregnancy but works mainly by inhibiting of ovulation (egg release). It may also delay embryo transport and make it more difficult for a fertilized egg to implant into the uterus by altering the endometrium.
Studies show that Plan B is fairly effective in preventing pregnancy after unprotected sex. Per the prescribing information for Plan B:
There is some controversy in regard to whether or not Plan B is effective at all after ovulation, but Plan B is nevertheless indicated for use at any time during the menstrual cycle.
It is important to be aware that Plan B use may cause spotting, bleeding or missed menses during the cycle Plan B was taken. If menses is late or doesn't occur, it would be prudent to take a pregnancy test or make a doctors appointment to rule out the possibility of pregnancy.
Dr. Brian Staiger is a licensed pharmacist in New York State and the founder of the Q+A website PharmacistAnswers.com. He graduated from the University At Buffalo with a Doctor of Pharmacy degree in 2010.

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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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