How Effective Is Plan B in People With Overweight?

Plan B One-Step, Next Choice, and Aftera are all brand names of levonorgestrel and good options for emergency contraception. But are they equally effective for everyone? Several studies suggest that levonorgestrel emergency contraception may be less effective, or not effective at all, for people with larger body sizes.Below, we explore how Plan B’s effectiveness can change for people with larger bodies.

Does Plan B have a weight limit?

The short answer is no. In the U.S., there is no weight limit for Plan B.

Health Canada, a regulatory agency, placed a warning on levonorgestrel products for their country, though. The labels warn of lower effectiveness in those who weigh more than 165 lbs and a lack of effectiveness in those who weigh more than 175 lbs.

Plan B effectiveness and weight

Women with larger bodies need to know that Plan B may be less effective, or not work at all, as emergency contraception.

Studies suggest that Plan B may be less effective in women who weigh 165 pounds to 176 pounds (75 kg to 80 kg) and even less effective in women who weigh over 176 pounds (80 kg) — especially those with a body mass index (BMI) at or above 30.

In one study, women taking Plan B who weighed over 176 pounds (80 kg) had the same pregnancy rate as those who did not use any emergency contraception. 

The same study found that women with a BMI at or above 30 were 4 times more likely to have pregnancy occur after taking Plan B than women with a BMI at or below 25. 

Generally, women with weight in this range are recommended to use an alternate form of emergency contraception, such as a copper intrauterine device (IUD) or ulipristal (Ella). Some experts say that if a person with heavier body weight is not able to receive a copper IUD or ulipristal, they can take a double dose of Plan B.

But, in the U.S., this is not common. Also, the only studies that looked at the safety and efficacy of a double dose have been very small. Larger studies would be needed to confirm that this is an appropriate option for emergency contraception. Never take a double dose of Plan B without your healthcare provider’s OK.

Why is Plan B less effective if you weigh more?

Experts don’t completely know why Plan B is less effective in larger bodies. There are several theories, but none of the studies were designed to test this relationship between weight and effectiveness.

Some of the theories as to why Plan B is less effective if you have a larger body size include:

• Those with larger bodies have more blood volume. It’s possible that the medication is more diluted in the larger blood volume, and that the medication does not have a strong enough effect.

• More weight may also mean more fat stores. Thus, some of the levonorgestrel hormone in Plan B can build up in fat cells. This causes a lower dose to get to the intended areas of the body.

• The medication may be broken down in different ways. Those with larger bodies may break down Plan B faster or slower, blunting the medication’s effects.

Some experts acknowledge that people with heavier body weights may just need a higher dose of Plan B for it to be more effective. But, as mentioned above, this has not been well-studied, and we don’t know the exact dose that would be needed — or if it would work.

Alternatives to Plan B for women with heavier weight

So, what other options are available for emergency contraception? Ella is more likely to be effective than levonorgestrel in people with heavier body weights.

Ella is a single-dose prescription emergency contraception that prevents ovulation by attaching to your body’s progesterone receptors (chemical binding sites). It prevents your natural progesterone from attaching to these areas and delays ovulation. It works differently than Plan B and is more effective in larger bodies.

But Ella can also be affected by body weight. One study found that Ella may be ineffective for those with a BMI at or above 35. 

As mentioned, people with heavier body weights can consider a copper IUD as an effective option for emergency contraception. A copper IUD does not seem to be affected by body weight.

There are some difficulties in using Ella or a copper IUD as emergency contraception, though. One of the main challenges for both of these medications is getting access to them. Ella may not be readily available at pharmacies. Also, the copper IUD needs to be placed during the short time period following unprotected sex. This may be difficult for some who don’t have quick access to a healthcare provider.

Always ask a healthcare provider which type of emergency contraception may be right for you.

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Written by:

Austin Ulrich, PharmD, BCACP

Austin Ulrich, PharmD, BCACP, is a board-certified ambulatory care clinical pharmacist. In his early career, he provided direct patient care in a variety of settings, including hospital and community pharmacies, and in a primary care clinic as a clinical pharmacist.

Edited by:

Samvida Patel, MNSP, INHC

Samvida Patel, MNSP, INHC, is a health editor at GoodRx. She is a nutritionist and integrative nutrition health coach with over 8 years of experience in health communications.

Reviewed by:

Christina Aungst, PharmD

Christina Aungst, PharmD, is a pharmacy editor for GoodRx. She began writing for GoodRx Health in 2019, transitioning from freelance writer to editor in 2021.

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