The morning-after pill may not be not effective for all women, as its European manufacturer is warning, although the American versions won’t carry the same alert.

The European version of the Plan B emergency contraceptive pill is completely ineffective for women who weigh more than 176 pounds and begins to lose effectiveness for women weighing more than 165 pounds.

The France-based HRA Pharma is changing its packaging to reflect the weight limits, but American consumers won’t receive similar warnings – although versions of the drugs sold in the U.S. are similar in dosage and chemical makeup to the European drug.

According to the Centers for Disease Control, the average American woman weighs 166.2 pounds.

"There's a whole swath of American women for whom (these pills) are not effective," says James Trussell, a professor of public affairs at Princeton University and a senior fellow with the Guttmacher Institute.

The pills use a compound called levonorgestrel to prevent pregnancies and are the most effective emergency contraceptive medication available without a prescription.

According to the CDC, 5.8 million American women used emergency contraceptive pills between 2006 and 2010.

American manufacturers do not currently warn American women of weight limits for the emergency contraceptives, and the Food and Drug Administration does not allow generic drug manufacturers to change product information unless the brand name manufacturer changes.

Drug manufacturers say that making a drug that’s effective for heavier women isn’t as simple as increasing the dosage.

"A dose increase of levonorgestrel is not proven to be a solution for this problem," said Karina Gajek, a spokeswoman for HRA Pharma.

She advised heavier women to discuss IUD insertion, which can cost up to $900, or alternative emergency contraceptives with their physicians.

[Image via Agence France-Presse]