Does Plan B Work After 72 Hours?

In our latest question and answer, the pharmacist discusses whether or not Plan B (levonorgestrel) is effective in preventing pregnancy after 72 hours (3 days).


When I had unprotected sex I might've already ovulated a day before or on the day of. Took a pill 3 days later, will the pill be effective?

Asked by Haylay On Feb 22, 2018

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Feb 26, 2018

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 if used with 72 hours of unprotected sex. However, data suggests that it may be be successful in preventing pregnancy for up to 120 hours (5 days), although efficacy is certainly higher the sooner Plan B is used.

Plan B Effects Over Time

We definitively know that Plan B losses effectiveness over time and the longer you wait after unprotected sex to take the pill, the less likely it is that the pill will work.  Based on available data, Plan B is most effective if taken within the first 72 hours after unprotected sex but is still somewhat effective up to 5 days (120 hours) after unprotected sex.

As mentioned, the sooner you take Plan B, the better. A large study by the World Health Organization states:

"Delaying levonorgestrel administration until the fifth day after unprotected intercourse increases the risk of pregnancy over five times compared with administration within 24 h."

It is important to note that it is unlikely that Plan B interrupts early pregnancy once implantation of a fertilized egg has already occurred. If an egg has had time to be fertilized and implant in the uterine wall, taking Plan B after this point will likely have no effect. In other words, Plan B will not affect an existing pregnancy.

How Plan B Works

Plan B (levonorgestrel) is thought to work via multiple mechanisms to prevent pregnancy, including:

  • Inhibition of ovulation
  • Inhibition or delay of embryo transport (inconclusive)
  • Impaired implantation into the uterus (unlikely)

Inhibition Of Ovulation & Associated Mechanisms

Many studies indicate that time of ovulation is the major factor in determining whether or not Plan B will be effective. Much of the contraceptive mechanism of Plan B is inconclusive but we do know that preventing ovulation is the primary mechanism of action. In fact, some studies suggest that Plan B is ONLY effective due its effect on ovulation.

Not all studies agree on this though and at the very least, the mechanism of contraception behind Plan B is complex. 

One of the more comprehensive studies on the matter states the following regarding Plan B use before ovulation:

"In conclusion, levonorgestrel emergency contraceptive [such as Plan B] administration during the pre-ovulatory days, the most fertile in the cycle, cannot prevent ovulation or fertilization with a dominant pre-fertilization [mechanism of action], but can be demonstrated to impair luteal function and may adversely affect the survival of the embryo"

In other words, in situations where ovulation was not prevented when Plan B was taken before ovulation, there were other mechanisms responsible for the contraceptive ability of Plan B.

Other Mechanisms 

Some of the most recent data suggests that Plan B has no little to no effect if used after ovulation and pregnancies occur at an expected rate, even when Plan B was taken soon after unprotected sex. However, many women have irregular ovulation cycles and it can be difficult to determine when ovulation occurred.

In addition, although most evidence in trending towards Plan B having no post-ovulation effects, there is still some grey area. One study puts it well:

"The gap between effectiveness of [Plan B] estimated from clinical studies and what can be attributed to disruption of ovulation may be explained by overestimation of actual effectiveness and supplementary mechanisms of action, including postfertilization effects. Additional data ... would yield greater insight into effectiveness and mechanisms of action."


  • Plan B (levonorgestrel) works mainly by preventing ovulation but may have multiple mechanisms behind its contraceptive efficacy.
  • Plan B is most effective when taken as soon as possible after unprotected sex.
  • Contraceptive efficacy significantly decreases after 72 hours but has still be shown to be effective up to 120 hours (5 days) after unprotected sex.
  • Efficacy of Plan B in regard to ovulation is inconclusive but likely is significantly more effective when used pre-ovulation.
  • Plan B is unlikely to affect a fertilized egg that has implanted into the uterine wall.

About the Pharmacist

Dr. Brian Staiger Pharm.D

Dr. Brian Staiger is a licensed pharmacist in New York State and the founder of He graduated from the University At Buffalo with a Doctor of Pharmacy degree in 2010. He has been featured in numerous publications including the Huffington Post as well as a variety of health and pharmacy-related blogs. Please feel free to reach out to him directly if you have any inquiries or want to connect! He's answered thousands of medication and pharmacy-related questions and he's ready to answer yours! Office: 716-389-3076

Recent Questions