Yes — for the most commonly available morning-after pill (levonorgestrel, sold as Plan B, I-pill, and similar brands), effectiveness does appear to reduce as body weight goes up, and research suggests it may not work as reliably above a certain weight. This isn't true of every emergency contraception option, so body weight is worth factoring into which one you choose, not a reason to skip emergency contraception altogether.

What the evidence actually shows

  • Levonorgestrel (the most common morning-after pill) — studies indicate its effectiveness starts to decline at higher body weights, and some data suggests it may be substantially less reliable in that range; it isn't that it "stops working," but that the odds of it failing go up
  • Ulipristal acetate (a prescription-only option in some countries) — appears to hold its effectiveness across a wider weight range than levonorgestrel, though it too may become less reliable at very high body weights
  • The copper IUD — the most effective emergency contraception option overall, and its effectiveness doesn't depend on body weight, because it works mechanically rather than hormonally
  • Global guidance on an exact weight or BMI "cutoff" isn't settled — different studies and regulators haven't agreed on one precise number, which is exactly why this is a conversation to have with a doctor rather than a self-check against a chart

What this means in practice

If you know your body weight falls on the higher end, taking a levonorgestrel pill later doesn't fix the issue — the concern is about how well it works at your body weight, not how soon you took it (though taking it as soon as possible still matters for anyone). Two practical options are worth knowing about: asking a doctor or pharmacist whether ulipristal acetate is available and appropriate for you, or considering a copper IUD inserted within 5 days, which is both the most effective option and unaffected by body weight.

Emergency contraception is still worth taking regardless of body weight — it lowers pregnancy risk compared to taking nothing. But if you can access it, a copper IUD or a conversation about ulipristal acetate is a more reliable choice at higher body weights than defaulting to a second levonorgestrel dose.

Frequently asked

Should I just take two morning-after pills if I weigh more?

No — doubling the dose isn't a validated way to restore effectiveness and can increase side effects like nausea without good evidence that it improves protection. If you're concerned about effectiveness at your body weight, ask about ulipristal acetate or a copper IUD instead.

Is there an exact weight cutoff where levonorgestrel stops working?

There isn't a single agreed number — studies point to reduced effectiveness at higher body weights, but regulators and researchers haven't settled on one universal cutoff. This is best discussed directly with a doctor or pharmacist who can weigh your specific situation rather than relying on a fixed chart.

How soon does emergency contraception need to be taken?

Sooner is always better — levonorgestrel is most effective within 72 hours, though it's sometimes used up to a bit later with reduced effect, and a copper IUD can be inserted up to 5 days after unprotected sex with high effectiveness throughout that window. If several days have already passed, a copper IUD is the option to ask about first.

Does this affect regular birth control pills too?

This specific weight-related effectiveness concern is best established for emergency contraception, particularly levonorgestrel. If you have concerns about your regular contraception and body weight, that's a separate conversation worth having with your doctor about the method that suits you best.