Does levonorgestrel (Plan B) remain effective in preventing pregnancy if administered during ovulation?

From the Guidelines

Plan B (levonorgestrel) is less effective if a person is currently ovulating, with its effectiveness significantly reduced if ovulation has already occurred. Plan B primarily works by preventing or delaying ovulation, so if ovulation has already occurred, its effectiveness is reduced. During ovulation, Plan B's effectiveness may drop to around 50% or lower, compared to its usual 87-95% effectiveness when taken within 72 hours of unprotected sex 1.

How Plan B Works

• Preventing or delaying ovulation is the primary mechanism of Plan B
• Secondary mechanisms, such as thickening cervical mucus or altering the uterine lining, may provide some protection, though these effects are less established 1

Alternative Emergency Contraception Options

• Ella (ulipristal acetate) may be more effective than levonorgestrel formulations after the first 72 hours and for women who are overweight or obese 1
• A copper IUD is the most effective emergency contraception (99.9%) and can be inserted up to 5 days after unprotected sex 1

Recommendation

If you're concerned about pregnancy during ovulation, consider more effective emergency contraception options like ella (ulipristal acetate) or a copper IUD. Plan B should be taken as soon as possible after unprotected sex (1.5mg levonorgestrel as a single dose), regardless of where you are in your cycle, as it may still provide some protection through secondary mechanisms.

From the FDA Drug Label

Levonorgestrel tablet works before release of an egg from the ovary. As a result, levonorgestrel tablet usually stops or delays the release of an egg from the ovary. The answer to whether Plan B still works if a person is currently ovulating is no, it is less likely to be effective because it works by stopping or delaying the release of an egg from the ovary.

• If ovulation has already occurred, levonorgestrel tablet may not be effective in preventing pregnancy.
• The drug label does not provide information on how to use levonorgestrel tablet if a person is currently ovulating, and it is recommended to talk to a doctor or pharmacist for further guidance 2.

From the Research

Effectiveness of Plan B During Ovulation

• Plan B, also known as levonorgestrel, is a form of emergency contraception that can be used to prevent pregnancy after unprotected sex 3, 4, 5.
• The effectiveness of Plan B in preventing pregnancy when taken during ovulation is still a topic of debate 5, 6.
• Studies have shown that Plan B is most effective when taken before ovulation, with a significant decrease in effectiveness when taken after ovulation 5, 6.
• One study found that when Plan B was taken after ovulation, the pregnancy rate was similar to what would be expected without any emergency contraception, suggesting that it may not be effective in preventing pregnancy when taken at this time 5.

Mechanism of Action

• The exact mechanism of action of Plan B is not fully understood, but it is thought to work by preventing or delaying ovulation, rather than preventing fertilization or implantation 4, 6.
• Some studies suggest that Plan B may have a post-fertilization effect, such as altering the luteal phase or decreasing progesterone levels, which could potentially prevent pregnancy 6.
• However, the evidence for this is still limited, and more research is needed to fully understand how Plan B works 6.

Comparison to Other Emergency Contraceptives

• Plan B is not the only form of emergency contraception available, and other options, such as ulipristal acetate, may be more effective in certain situations 3, 7.
• Ulipristal acetate, for example, has been shown to be effective in preventing pregnancy when taken up to 5 days after unprotected sex, and may be a better option for women who are already ovulating 3, 4.