Pfizer announced a recall of approximately 1 million birth control pill packs in the United States and warned users of the recalled pills to start using back-up methods immediately to prevent pregnancy.

The pills recalled are marketed under the name Lo/Ovral-28 and as generic Norgestrel/Ethinyl Estradiol as sold by Akrimax Pharmaceuticals. (A full list of lot numbers subject to recall is available here.) Pfizer told the Associated Press that tests revealed that quality control was at issue with the affected lots: "some packets of the drugs had too many active tablets, while others had too few."

While there is no safety risk to the pills, users with packets that contain placebos instead of active pills are effectively -- and unknowingly -- not protected against pregnancy while taking those pills, and potentially for the month after as their bodies readjust. Users are encouraged to tell their doctors and return the affected pills to their pharmacies.

More than 10 million American women use hormonal birth control pills for contraception, and it's the most common method of contraception among women under 30. More than half of women who have abortions report that they got pregnant due to contraception failure.