Pfizer found that some packets of the drugs had too many active tablets, while others had too few. Pfizer said the issue was a result of mechanical and visual inspection failures.
The error doesn't pose any immediate health risks, but it could leave women with an inadequate dose of the hormone-based drugs and raise the risk of an unintended pregnancy, the company said.
"This isn't a health risk to a woman, but it is a pregnancy risk, so you have to know you need another form of protection," said ABC News Chief Health Editor Dr. Richard Besser.
The problem affects 14 lots of Lo/Ovral-28 tablets and 14 lots of generic Norgestrel and Ethinyl Estradiol tablets. The packs have expiration dates ranging between July 31, 2013 and March 31, 2014.
Besser said it's unknown how long these pills have been on the market.
"I think they have some explaining to do," Besser said.
Patients who have the affected product should immediately notify their physician and return the product to the pharmacy.
For lot numbers of the affected packs and contact information, visit Pfizer's website, www.Pfizer.com.
Pfizer says the problem has been corrected. The drugs were distributed to warehouses, clinics and retail pharmacies throughout the U.S.
Birth control pills in general are supposed to be 99-percent effective, and if they fail, it's usually because a woman forgets a dose. Besser said this incident should remind women to be precise with their medication.
The Associated Press contributed to this report.
