Some studies suggest that the active ingredient in Plan B may work less effectively at standard doses in people with a body weight of over 155 pounds (70.3 kilograms). However, individuals above this body weight can still take it.

Plan B contains the hormone levonorgestrel. According to trials, Plan B One-Step can prevent 84% of pregnancies after unprotected sex.

The advocacy group Planned Parenthood recommends that people with a higher body weight consider other emergency contraception methods, such as the copper intrauterine device (IUD). Scientists suggest these are just as effective in all people, regardless of body weight.

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Plan B does not have a weight limit that prevents people with a higher body mass index (BMI) from taking it. It is still safe to use in individuals with a higher body weight, and it will still substantially reduce the chance of pregnancy when a person uses it within 3 days of having unprotected sex.

However, Planned Parenthood recommends that people who weigh over 155 pounds (lb), or 70.3 kilograms (kg), consider other methods of emergency contraception. This is due to studies suggesting that body weight may affect how well levonorgestrel works at standard doses.

Research has found an association between having a higher BMI and an increased number of pregnancies among those who take levonorgestrel to prevent pregnancy.

A 2015 study found that, among people taking emergency contraceptives, the chance of becoming pregnant began increasing at body weights of 155 lb (70.3 kg) and rose to a pregnancy rate of 6.4% at 176 lb (79.8 kg). The rate of pregnancy for individuals of lower weights was 1.4%.

Additionally, according to a 2016 review, a person has around a 6% chance of conceiving if they have sex without any contraception, so this difference in pregnancy rate is substantial. Another analysis the authors looked at found that the likelihood of pregnancy was four times higher in people with a BMI of 30 compared to those with BMIs under 30.

However, not all research has found such significant differences in levonorgestrel’s effectiveness. A 2017 review of four randomized trials that included 6,873 women found that while pregnancy rates differed between those with higher and lower BMIs, the risk was lower than in these previous studies.

The researchers found that after taking 1.5 milligrams of levonorgestrel, people with a BMI of 30 or more had a 2% rate of pregnancy, compared to an average rate of 1.2% across all participants.

Scientists are not sure why body weight seems to affect levonorgestrel’s impact — it may relate to the concentration of levonorgestrel in the blood.

This has prompted some health organizations to suggest that doctors administer higher doses of levonorgestrel to people above a certain weight. However, there is no research yet to confirm this is safe or effective.

Plan B still provides significant protection against pregnancy, regardless of body weight. Even studies that find a difference in its effectiveness find that the overall pregnancy rate is low in individuals who use it correctly.

Therefore, if a person with a higher BMI has taken Plan B to prevent pregnancy, there is still a high chance it will work.

Plan B is one of several options for emergency contraception. Other options may be more effective in people with a higher BMI.

Ulipristal acetate

Similar to Plan B and other levonorgestrel-containing pills, ulipristal acetate prevents ovulation, which reduces the chances of pregnancy. There is one brand of this medication, which is Ella.

Ella is the most effective type of morning-after pill and remains as effective in people weighing up to 195 lb (88.4 kg). However, unlike Plan B, Ella requires a prescription from a doctor or nurse.

Ella can also contaminate breast milk, which means that a lactating person must discard their milk for about 36 hours after taking it.

Learn more about the Ella morning-after pill.

Copper IUD

The copper IUD is the most effective method of emergency contraception. A 2018 review argues that it is also the most suitable option for those with higher BMIs, because it works just as well in people of all sizes.

A copper IUD is a small device that a medical professional inserts into a person’s uterus. It is both an emergency contraceptive and a long-acting form of birth control.

People can use copper IUDs, such as Paraguard, within 120 hours, or 5 days, of having unprotected sex. A person will need to make an appointment with a healthcare professional for fitting.

The procedure can cause some pain or bleeding after insertion, and the copper IUD can also affect periods, making them heavier. However, this does not happen to everyone, and the side effects can decrease over time.

People can keep the copper IUD for up to 12 years before needing to replace it. As a general contraceptive, Paraguard is 99.9% effective against pregnancy from day one.

Learn more about the copper IUD.

Oral emergency contraception works by preventing ovulation, while the copper IUD works by making the lining of the uterus unreceptive to implantation. While there are at-home kits to predict ovulation, these do not provide a reliable indication of whether emergency contraception has worked. The only reliable way to know is to take a pregnancy test when the next period is due.

For most people, this will mean taking a pregnancy test around 2 weeks after ovulation. A negative pregnancy test suggests but does not prove that emergency contraception worked.

A person does not need a prescription or a doctor’s permission to take Plan B. However, it is a good idea to talk with a medical professional if an individual:

  • needs emergency contraception, but they are above 155 lbs (70.3 kg) in weight
  • has already taken Plan B and has concerns about its effectiveness
  • experiences severe or long-lasting side effects of emergency contraception
  • believes they could be pregnant

Plan B is an emergency contraceptive that contains levonorgestrel. Some research suggests that a higher body weight may reduce how effective standard doses of levonorgestrel are for preventing pregnancy. However, while several studies have shown this, the extent to which body weight affects levonorgestrel’s effectiveness is unknown.

Planned Parenthood recommends that people with a body weight above 155 lbs (70.3 kg) use either Ella or a copper IUD, rather than Plan B. However, this does not mean that individuals over this weight cannot take Plan B or that it will not work.

No emergency contraceptive can prevent 100% of all pregnancies. Even in people with lower body weight, the effectiveness of Plan B is around 84%. Individuals who have concerns about whether Plan B is suitable for them should speak with a doctor or sexual health professional.