Plan B becomes less effective as body weight increases, and 165 pounds (about 75 kg) is roughly where that decline becomes significant. The pill doesn’t suddenly stop working at that exact number, but pregnancy rates rise sharply in that weight range, and by around 176 pounds (80 kg), some analyses suggest levonorgestrel may offer little to no emergency contraceptive benefit at all. More effective alternatives exist for people in this weight range.
Where the 165-Pound Number Comes From
In 2013, European regulators changed the label on a levonorgestrel emergency contraception product to warn that it was less effective in women weighing 165 pounds or more and ineffective in women over 176 pounds. This generated widespread media coverage, especially in the U.S. The following year, the European Medicines Agency reversed that label change, and in 2016 the FDA determined a similar warning wasn’t warranted for American products, partly because the available data had limitations and regulators didn’t want to discourage use entirely.
So the 165-pound threshold isn’t on the box, but the science behind it is real. The reversal was more about regulatory caution than a finding that weight doesn’t matter. Multiple pooled analyses consistently show a meaningful drop in efficacy starting around that weight.
How Much Effectiveness Drops
The numbers tell a clear story. Among women with a BMI under 25 (roughly under 150 pounds for an average-height woman), the pregnancy rate after taking levonorgestrel emergency contraception is about 1.3%. For overweight women (BMI 25 to 30), it rises to 2.5%. For women with a BMI of 30 or higher, it jumps to 5.8%.
When researchers looked at weight directly rather than BMI, the pattern was similar. Women weighing 143 to 165 pounds had a pregnancy rate of about 1.4%. Between 165 and 187 pounds, that rate climbed to roughly 6%. For context, the estimated chance of getting pregnant from a single act of unprotected sex without any contraception is around 5 to 6%, meaning Plan B may provide minimal additional protection at higher weights.
Women who became pregnant despite taking levonorgestrel weighed significantly more on average (169 pounds) than those who didn’t become pregnant (146 pounds). Women with obesity had more than four times the odds of pregnancy compared to women at lower weights.
Why Weight Reduces Effectiveness
Plan B works primarily by delaying or preventing ovulation. For that to happen, levonorgestrel needs to reach a high enough concentration in the blood quickly enough. In women with obesity, peak blood levels of the drug were roughly half those seen in normal-weight women (5.57 vs. 10.30 ng/mL). That’s a dramatic difference for a time-sensitive, single-dose medication.
The exact mechanism isn’t fully mapped out, but higher body mass affects how the drug is absorbed, distributed through tissues, and bound to proteins in the blood. The result is that less active drug reaches the places it needs to go, and it gets there more slowly. Since emergency contraception is a race against the clock before ovulation occurs, that delay can be the difference between the pill working and not working.
Would Taking Two Pills Help?
It’s a logical question: if the problem is lower drug levels, why not double the dose? Researchers have studied this. A double dose (3 mg instead of the standard 1.5 mg) did raise blood levels in women with obesity, but not to the same peak concentrations seen in normal-weight women taking a single dose. The clinical evidence so far hasn’t been strong enough to confirm that doubling up meaningfully improves pregnancy prevention. Some clinicians still mention it as an option when nothing else is available, but it’s not a well-supported strategy.
Alternatives That Work Better at Higher Weights
If you weigh more than 165 pounds, two options perform significantly better than Plan B.
- Ella (ulipristal acetate): This prescription morning-after pill works better than Plan B for people over 165 pounds. It remains effective up to about 195 pounds and works up to five days after unprotected sex, compared to Plan B’s three-day window. It does require a prescription or a telehealth visit in most cases, which adds a step. Above 195 pounds, ella’s effectiveness also starts to decline.
- Copper IUD: This is the most effective form of emergency contraception available, with a failure rate under 1% regardless of body weight. It can be placed up to five days after unprotected sex and then doubles as long-term birth control for up to 10 years. The tradeoff is that it requires a clinic visit for insertion, which isn’t always possible on short notice.
Weight has no impact on the copper IUD’s effectiveness. For people over 195 pounds, it is the only emergency contraceptive option with strong evidence of full efficacy.
What This Means Practically
Plan B is available over the counter without a prescription, which makes it the fastest option in an urgent situation. Guidelines from major medical organizations still say it should not be withheld based on weight, because some protection is generally considered better than none, and individual responses vary. But “don’t withhold it” is different from “it works equally well for everyone.”
If you’re over 165 pounds and need emergency contraception, the most effective path is a copper IUD if you can get to a provider quickly. If that’s not realistic, ella is the next best choice. If neither is accessible, taking Plan B is still reasonable, but it’s worth understanding that your odds of it preventing pregnancy are meaningfully lower than they would be for someone who weighs less. The steep drop in effectiveness between 155 and 175 pounds is one of the most consistent findings in emergency contraception research, even if regulators have been cautious about putting it on the label.