Around one million packets of birth control tablets are being recalled in the US, as they might not prevent pregnancy.
The pharmaceutical company Pfizer said a "packaging error" meant the doses were not correct.
It said the tablets did not pose any health dangers, but there was a risk of "unintended pregnancy".
Pfizer is advising women affected to use non-hormonal forms of contraception immediately.
Fourteen lots of Lo/Ovral-28 tablets and 14 lots of Norgestrel and ethinyl estradiol tablets have been recalled. The lot numbers have been published on the company's website.
Throughout the month women would take 21 tablets containing active ingredients and seven which were inert.
The correct dose is essential for preventing pregnancy, however, the drug maker said: "The daily regimen for these oral contraceptives may be incorrect and could leave women without adequate contraception, and at risk for unintended pregnancy."
Women affected have been advised to tell their doctor and return the tablets to their pharmacy.
