Plan B has a surprisingly low weight limit - here are the other emergency contraception options

• Plan B may be less effective if you are over 155 pounds or have a BMI over 25.
• More research is needed to determine the impact of weight on Plan B.
• If you are over 155 lbs you should use another emergency contraception like Ella or the copper IUD.

Emergency contraceptives, like the morning after pill, can help prevent pregnancy even after you've had unprotected sex. However, research indicates these morning-after pills may not be effective for those who are above a certain weight.

There are three main types of emergency contraception, each of which is influenced by weight differently:

• Plan B, or pills with the hormone Levonorgestrel
• Ella, a non-hormonal morning after pill
• Copper IUD, a non-hormonal form of birth control

Here's what you need to know about weight limits when using emergency contraception, and what other options may be available to you.

Plan B, weight, and BMI

Plan B may be less effective if you weigh more than 155 lb or have a BMI over 25.

Some researchers believe that weight and BMI - which stands for body mass index - alters the effectiveness of Plan B because those with more fat cells metabolize drugs differently than people with lower fat stores, says Rachel Bowman, MD, an assistant professor at The University of Texas Austin Dell Medical School.

"However, this research is limited and not well explored," Bowman says.

What the research says

It's still unclear as to whether morning-after pills are less effective for people above a certain weight, as the research has presented mixed results.

Here's a survey of what scientific research has indicated:

• A large 2015 study found as body weight and BMI increases, the effectiveness of levonorgestrel-based emergency contraception methods, like Plan B, decreases. Women with higher BMIs had a 6.4% rate of unintended pregnancy whereas women with lower BMIs had a 1.4% rate.
• A 2016 review found that obese women who took levonorgestrel morning-after pills had lower concentrations of the drug in their bodies than those with a healthy BMI. This means the drug wasn't metabolized properly in those with higher BMIs. The study also found the morning-after pill was four times more likely to fail in obese women.
• However, a 2017 review analyzed the effect of BMI and body weight on emergency contraception effectiveness and found that pregnancy rates were less than 3% for levonorgestrel morning-after pills in women with both normal and high BMIs. In this case, weight and BMI didn't impact the effectiveness of Plan B.

Should I still take Plan B if I'm over the weight limit?

Even if you are above the recommended weight limit for Plan B, you can still take it.

"There are really no medical contraindications to taking emergency contraceptives, and the benefit to them is potentially higher than any potential risks - pregnancy could be a much riskier state than taking a single dose of either medication," says Staci Tanouye, MD a gynecologist based in Jacksonville, Florida.

Although Plan B is safe and there are no serious or long-term side effects, some people may experience short-term side effects, says Bowman.

These can include:

• Headaches
• Nausea
• Fatigue
• Spotting

How to choose the best type of emergency contraception for your body

There are other emergency contraception options that can be more effective than Plan B, especially if you are over 155 lbs.

General advice: The copper IUD is the best emergency contraceptive option for women above a BMI of 35. "BMI does not alter the effectiveness of the copper IUD," says Simon.

Here's a breakdown of the different emergency contraception options, and which is best for you based on your weight.

Insider's takeaway

Plan B has a weight limit of 155 lbs, but it's still safe to take morning-after pills even if you fall above the recommended weight limit. Although there's a chance they might not be as effective, it's not likely to pose any health risks.

When it comes to emergency contraception options, the IUD is the most effective option for people of all weights. The best advice is to communicate with your gynecologist or healthcare provider to determine which contraceptive option is right for you.

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