In Europe, a morning-after pill identical to Plan B is going to come with a warning— the manufacturer has found the drug is ineffective for women who weigh more than 176 pounds, and begins to lose effectiveness in women who weigh more than 165 pounds. The average American woman weighs 166.2 pounds

And its unclear whether women in the U.S. will know about this warning even though it applies to them. Mother Jones' Molly Redden reports: 

HRA Pharma, the French manufacturer of the European drug, Norlevo, is changing its packaging information to reflect the weight limits. European pharmaceutical regulators approved the change on November 10, but it has not been previously reported.

... Some of the most popular emergency contraceptive pills sold over-the-counter in the United States—including the one-pill drugs Plan B One-StepNext Choice One Dose, and My Way, and a number of generic two-pill emergency contraceptives—have a dosage and chemical makeup identical to the European drug.

In short: there may be women who are using a morning-after pill that's not working the way it should be, ergo, not preventing a pregnancy the way it should be. Further, it seems that Plan B, Norlevo, and their generic counterparts do as much for heavier women's contraception as a glass of water.

If these studies are correct (they seem sturdy enough for a drug company to issue a warning) is particularly troubling for a large number of American women. According to numbers compiled by the CDC, the average American woman over the age of 30 would fall under the pill's warning of diminishing ineffectiveness (the number on the right is the weight of the women in pounds): 

And the average non-hispanic white woman over the age of 20 also falls into that group:

Further, the average weight for black American women is, under the warning, considered too high for the pill to work at all: 

Because of a hitch in the FDA rules, Americans won't get the same warning label ("Studies suggest that Norlevo is less effective in women weighing [165 pounds] or more and not effective in women weighing [176 pounds] or more") that European women are getting, even though the pills are chemically the same. 

Redden reports:

Because the Food and Drug Administration prohibits generic drug manufacturers from changing product information unless the brand name manufacturer makes a change, companies that manufacture generic versions of Plan B One-Step cannot update their packaging information unless Teva Pharmaceutical Industries, the exclusive manufacturer of Plan B One-Step, acts first.

Teva and a slew of other drug manufacturers didn't speak to Redden, but it would seem like these companies should do something (or anything) like proving the studies wrong, or putting a precautionary warning out there, or something that isn't just sitting around with the possibility of an ineffective, very important drug out there on the market.