The Food and Drug Administration (FDA) has said that the morning-after pill may not work for women who weigh more than 176 pounds, based on information out of Europe.

HRA Pharma, the French manufacturer of Norlevo, an emergency contraceptive pill identical to Plan B (aka the morning-after pill), found that its product began to lose effectiveness for females above 165 pounds, and was completely ineffective for those weighing more than 176 pounds.

The news could mean that millions of American women won’t be able to use Plan B or its generic equivalents. The average weight of females 20 years and older is 166.2 pounds, according to data collected by the Centers for Disease Control and Prevention (CDC) from 2007 to 2010.

HRA Pharma is now changing its packaging information to warn women about the new weight limits.

The finding impacts American versions of the contraception, including Plan B One-Step and generics Next Choice One Dose and My Way, because many use the same key ingredient (levonorgestrel) and dosage as Norlevo.

The producers of Next Choice and My Way can’t update their labeling to reflect the new weight limits until Plan B’s maker, Teva, does so first. This is due to a quirk in federal law, which the FDA seeks to change.

It was unclear what Teva plans to do. A company spokeswoman declined to comment when contacted by Mother Jones.

-Noel Brinkerhoff

To Learn More:

New Warning: Morning-After Pill Doesn't Work for Women Over 176 Pounds (by Molly Redden, Mother Jones)

If You Weigh More Than 176 Pounds, Plan B Might Not Work. What’s Plan C? (by Amanda Marcotte, Slate)

Judge Escalates Battle with Obama Administration over Morning-After Pill (by Matt Bewig, AllGov)