The FDA is investigating whether some morning after pills need a label change.
This comes after the European maker of the "Norlevo" emergency contraceptive reported that it does not work well in women who weigh 165 pounds or more.
The company also said the pill is "not effective" for women over 176 pounds.
Norvelo has the same active ingredient as Plan "B" one-step, the emergency contraceptive pill approved in the US.
The FDA is looking over the European studies to determine if the results apply to the US version of the pill.
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