This month’s very late Science of Sex post was motivated by the new Hulu show Shrill, which reminded the world that emergency contraception may not be as effective for overweight people. Specifically, the brand Plan B (also known as the morning after pill) EC is rated up to 175 pounds, which is just over the average weight of an American woman.

The results of a large European study show that the popular form of EC, levonorgestrel 1.5 mg, (Plan B pill) loses its potency in women weighing about 165 pounds and does not work at all in women weighing 175 pounds or more.

There are several theories as to why.

1. Having more fat makes pills break down more quickly. This makes the medication less effective.
2. Overweight people have more blood, so more birth control is required to reach ideal blood concentration levels.

Researchers haven’t pinpointed which theory, if either, is correct. All we know is that EC might not be reliable for many people. This was something I learned last year thanks to Reddit, but it’s not something that many people have heard from doctors, pharmacists, or other professionals. Even the manufacturer denies this to be the case.

Fortunately, there is another option: Ella. According to the study, Ella is more effective than Plan B for overweight users, and you can take the pill up to five days later. In comparison, Plan B loses effectiveness after 72 hours. However, you may need a prescription for Ella.

Ella bested Plan B by half, with about 50% fewer pregnancies than those taking levonorgestrel.

Additionally, you may be able to double up on doses of Plan B to increase its effectiveness if you are overweight.

This increased risk of pregnancy doesn’t end with EC. Cochrane examined different studies and found mixed results. The effectiveness of birth control didn’t decrease universally as BMI increased, but the patch and some birth control pills were less effective for overweight and obese users. Shots, implants, and hormonal IUDs were not less effective.

Across the board, IUDs are touted as being the most effective birth control or EC regardless of BMI.

But any contraception is better than none. The European Medicines Association advises that using Plan B is better for overweight people than using no EC. You just need to be aware of the risks.

Further Reading

• Does being overweight affect your birth control?
• Emergency contraception review: evidence-based recommendations for clinicians
• Pregnant Pause: “Plan B” & Overweight Women
• Are emergency contraceptive pills effective for overweight or obese women?
• Impact of obesity on the pharmacokinetics of levonorgestrel-based emergency contraception: single and double dosing.
• Emergency Contraception Algorithm and Guide for Clinicians
• The Truth About Plan B’s “Weight Limit”
• Hormones for birth control in overweight or obese women

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