"As a result of this packaging error, the daily regimen for these oral contraceptives may be incorrect and could leave women without adequate contraception, and at risk for unintended pregnancy," according to a Pfizer statement on the U.S. Food and Drug Administration website.
Pfizer found that some packets of the drugs had too many active tablets, while others had too few.
The drugmaker said the issue involved 14 lots of Lo/Ovral-28 tablets and 14 lots of Norgestrel and Ethinyl Estradiol tablets.
A company spokeswoman said the problem was caused by both mechanical and visual inspection failures on the packaging line, The Associated Press reported.
She said the problem has been corrected.
Image: Birth control pills, via Shutterstock.