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Plan B may be less effective if you weigh 155 pounds or more, or have a BMI of 30 or higher.

However, there are no safety concerns for using it if either applies to you. The Food and Drug Administration (FDA) hasn’t yet made any changes to the Plan B label or to its prescription recommendations.

Why? Because there isn’t enough research to demonstrate a solid link between weight and lowered Plan B effectiveness.

So, if you’re at or over this limit, you can still take Plan B.

However, another form of emergency contraception (EC) may prove to be a better option.

BMI is different than weight. It’s calculated by dividing a person’s weight by the square of their height.

Research has found that both a higher weight and BMI have a link to lowered effectiveness of EC containing levonorgestrel.

Plan B falls into that category of EC methods.

According to a preliminary study carried out in 2016, people with a BMI of 30 or higher had a significantly lower level of levonorgestrel EC in their bloodstream than people with a BMI between 18.5 and 25.

This suggests that Plan B may be less effective for people who have a BMI at or above 30. Hormone production that affects ovarian activity may be a reason why.

But experts still aren’t sure about the exact level of effectiveness — lowered or otherwise — that may result from these factors.

Research is limited in this area. In some cases, it’s produced conflicting results.

A 2011 study found that pregnancy risk was more than three times greater in participants who took a levonorgestrel-based EC and had a BMI of 30 or higher than for participants who had a BMI between 18.5 and 25.

But the findings weren’t strong enough to prove that a higher BMI was the cause of this drop in effectiveness.

Although further studies found a similar link, a 2017 research review concluded that, after taking levonorgestrel-based EC, the pregnancy rate was low across different BMI and weight categories.

This led the researchers to conclude that the likes of Plan B should “not be restricted in any weight or BMI category.”

Due to one study in their review that found an increase in pregnancy rates among people with a higher BMI, the researchers recommended more attention for counseling and advice for people in this group.

That said, it’s important to note that the data available for review was limited in both scale and diversity.

More research is needed to fully understand how levonorgestrel-based EC pills, as well as all others, may work for people who have a BMI of 30 or higher.

There are four kinds of EC:

  • a progestin-only pill containing levonorgestrel
  • a pill containing ulipristal acetate (UPA)
  • a copper IUD
  • an increased dose of certain combination birth control pills

As with Plan B and other levonorgestrel-based pills, the UPA option known as ella may be less effective for those with a higher weight.

Planned Parenthood states that ella may work less well for those who weigh 195 pounds or more.

However, ella may still be more effective for those with a higher weight or BMI than Plan B and other levonorgestrel-based pills.

The copper IUD, on the other hand, has the same level of effectiveness regardless of a person’s weight.

Brand nameWeight limitBMI limitWhen to take it
EC pill with ulipristal acetateellaup to 195 lbs.nonewithin 5 days
EC pills with levonorgestrel (a progestin)Plan B, Next choiceup to 155 lbs.up to 30within 72 hours
combination birth control pillsAviane, Enpresse, Levora, Nordette, Seasonale, and morenonenonewithin 72 hours
copper IUDParagardnonenonewithin 5 days

All EC methods work in a similar way.

They’re designed to prevent pregnancy by obstructing the likes of ovulation, fertilization, or implantation.

But when you should take it and how effective it is differs.

EC pill with ulipristal acetate

Unlike Plan B, the emergency contraceptive pill that contains UPA is only available via prescription.

Branded as ella, this pill blocks the normal functions of progesterone, preventing or delaying ovulation.

To work, ella must be taken within 5 days of sex. If taken correctly, it can reduce the risk of pregnancy by 85 percent.

But it doesn’t matter whether you take the pill on day 1 or day 5. Its effectiveness remains the same.

Ella is also more likely to inhibit impending ovulation than levonorgestrel-based EC pills. This makes it more effective than the likes of Plan B if you’re in your fertile window.

EC pills with levonorgestrel (a progestin)

There are a few kinds of progestin-only ECs, including Plan B and Next Choice.

They all contain a synthetic hormone called levonorgestrel. They’re all available over the counter.

Levonorgestrel delays or prevents ovulation by diminishing the release of luteinizing hormone, which sends a signal for an egg to be released.

If taken within 24 hours of sex, this type of EC pill can lower the risk of pregnancy by 95 percent.

If taken 24 to 72 hours after sex, it can reduce the risk of pregnancy by 88 percent.

Combined oral contraceptive pills

Although everyday birth control pills aren’t labeled for emergency use, it’s possible to take a specific number of the combination type to prevent a pregnancy.

The hormones they contain — estrogen and progestin — essentially do the same job as pills marketed as EC, but you’ll need more than the usual daily dose for this to work.

The Yuzpe method, as it’s known, was named after Professor A. Albert Yuzpe — the man who proved the effectiveness of this approach.

However, it’s the least effective option. If taken within 72 hours of sex, it can reduce the risk of pregnancy by 75 percent.

Almost 30 different combination birth control pills are currently approved for this method.

You’ll need to ensure you’re taking two doses of the correct number of pills, exactly 12 hours apart.

Copper IUD

A copper IUD is the most effective form of EC.

It lowers the risk of pregnancy by more than 99 percent by releasing copper into the uterus to stop fertilization or implantation.

The process is slightly more complicated than taking a pill, though.

A healthcare provider will need to insert the device into the uterus within 5 days of sex.

This can be costly and a little painful, but the upside is that a copper IUD can be left in for up to 12 years as a form of long-term contraception.

(You can learn more about getting the IUD inserted for free or at a low cost here.)

If you do want it removed, you’ll have to wait until after your next period to do so.

A copper IUD is more than 99 percent effective, regardless of a person’s weight.

This makes it the best option for anyone who’s concerned about the effectiveness of an EC pill.

The length of time that’s passed after sex and where you are in your menstrual cycle can both impact the effectiveness of EC pills.

Although some forms like ella remain just as effective whether they’re taken 1 or 5 days after sex, others — like Plan B — are more effective the sooner they’re taken.

Plus, the closer you are to ovulation when having sex, the more likely it is that the pill’s effectiveness will be affected.

There are a number of other factors to consider, too.

Certain medications and herbal remedies have been known to interact with EC pills.

The list currently includes:

  • St. John’s wort
  • antibiotics, such as rifampicin and rifabutin
  • medications that make the stomach less acidic
  • some medications that treat epilepsy, tuberculosis, or HIV

If you’re concerned that you may be affected by one or more of the above, talk with your healthcare provider before taking an EC pill.

It’s also important to seek medical advice if you’re taking EC due to forgetting to take another form of birth control. In these instances, ella may not work as well as Plan B.

The same applies if you vomit within a few hours of taking any pill, as your body may not have had time to absorb it.

Doubling up on the dose won’t make EC pills any more effective, but it can give you side effects like nausea.

To ensure the highest level of effectiveness, take EC within the recommended time frame.

For Plan B and other levonorgestrel pills, this means taking it within 72 hours of sex. Their effectiveness is highest when taken within 24 hours.

Ella can be used anytime within 5 days of sex, and so can the copper IUD.

If you’re using combined birth control pills, ensure you take both doses within 72 hours.

It’s difficult to know whether EC hasn’t worked. In fact, the only foolproof way is to wait for your next period.

If you know when your period is due and it appears to be more than 7 days late, take a home pregnancy test.

If you’re not sure when your period’s due, wait at least 21 days after sex to take a test.

The hormone that pregnancy tests detect is only produced 6 days after fertilization, so there’s no point in taking one early.

If a pregnancy test comes back positive, book a doctor’s appointment for advice on all your options, including maintaining or ending the pregnancy.

Be aware, too, that vomiting within a couple of hours of taking any form of pill can result in it not working.

If that’s the case, you may need to take another dose. Just make sure you talk with a healthcare provider beforehand.

Weight and BMI may lower the effectiveness of Plan B, but much more research is needed to confirm this.

Right now, though, there’s no reason not to take Plan B if you need it.

If you’re concerned about its effectiveness, talk with a doctor or other healthcare provider to learn more about the best option for you.

And remember: Just because something could be less effective doesn’t mean that it won’t work at all.

Lauren Sharkey is a U.K.-based journalist and author specializing in women’s issues. When she isn’t trying to discover a way to banish migraine, she can be found uncovering the answers to your lurking health questions. She has also written a book profiling young female activists across the globe and is currently building a community of such resisters. Catch her on Twitter.