WEDNESDAY, Feb. 1 (HealthDay News) -- About one million packets of birth control pills are being recalled in the United States by Pfizer Inc. because of a packaging error that could reduce the effectiveness of the pills and cause unintended pregnancies.

The recall of 14 lots of Lo/Ovral-28 tablets and 14 lots of generic Norgestrel and Ethinyl Estradiol tablets was announced late Tuesday after Pfizer discovered that some blister packs may contain either too many or too few active pills and that the pills may be out of sequence. The expiration dates on the recalled packets range between July 31, 2013 and March 31, 2014.

The pills are packaged in blister packs containing 21 active tablets and seven inactive tablets. The packaging error means that the daily regimen for the pills may be incorrect and could leave women without adequate contraception and at risk for unintended pregnancy, Pfizer said in a news release.

Experts said the error could pose real problems for women using the pills.

"This is extremely important," said Dr. Steven Goldstein, professor of obstetrics and gynecology at NYU Langone School of Medicine in New York City. "It is absolutely essential that birth control pills be taken as intended, with 21 medicated pills in a row. Otherwise ovulation could take place and an unintended pregnancy is definitely a possibility."

Another expert agreed.

"Sequence and content of active versus inactive pills is crucial in determining efficacy, including whether or not unwanted pregnancy will be actually prevented," said Dr. Jill Rabin, chief of ambulatory care, obstetrics and gynecology, head of urogynecology at Long Island Jewish Medical Center in New Hyde Park, N.Y.

She said that women who determine that they do have the recalled contraceptive pills should first "ensure that you are not currently pregnant (by taking a pregnancy test) if you have any symptoms of pregnancy (missed periods/abnormal bleeding, etc)." They should also switch immediately to a non-hormonal form of contraception if they are not pregnant, notify their health care provider of the situation, and return the defective product to the pharmacy.

Pfizer has informed the U.S. Food and Drug Administration about the recall. In a statement issued Wednesday, the FDA agreed that "patients who have the affected product should notify their physician and return the product to the pharmacy."

The cause of the error was identified and corrected immediately, according to Pfizer.

The birth control pills were manufactured and packaged by Pfizer and sold under the Akrimax Pharmaceuticals brand. The pills were distributed to warehouses, clinics and retail pharmacies across the United States.

More information

Here's where you can find more details on lot numbers and other information on pills inc...ed in the recall.