INDIANAPOLIS -- Birth control pills are known to be nearly 100 percent effective when taken properly, but a recall of the drugs could send a shudder through women of childbearing age.

A manufacturing mix-up by Pfizer, the world’s largest drug maker, led to some packets being distributed with the pills out of order. That means a patient could have unknowingly skipped a dose and raised her risk of an accidental pregnancy.

Pfizer has recalled about one million packets of Lo/Ovral-28 and its generic equivalent, but the company estimates that only about 30 packets were flawed. The pills were made and shipped last year.

Each packet contains 28 days’ worth of the prescription, with 21 pills containing the active ingredient that prevents pregnancy and seven placebo pills. The pills are normally color-coded to note the difference. Women are supposed to take the pills in order and not mix the placebos with the active pills. Doctors say taking three placebos or more in a row negates the pregnancy protection.

“Women who are potentially on this particular product need to be aware of [the recall] and take this quite seriously,” said Dr. Steven Goldstein, a New York University professor of obstetrics and gynecology.

The packaging mistake worries Kendra Mifflin even though she takes a different brand of the pill.

“You take it for a reason, and if that gets messed up … it could change your life,” she said. “It has a big impact, just a simple mistake like that.”

The student at Butler University in Indianapolis said her pills change color as she gets closer to the placebo. Mifflin, who has been taking them for about six months, said she thinks she would notice if the color was out of order, but she’ll probably pay closer attention now.

The drug is not among the more commonly prescribed brands of birth control. Doctors and pharmacists say Lo/Ovral is an older version of the pill that is not as widely prescribed anymore. They do not expect many panicked calls about the mix-up.

“Patients are pretty savvy,” said Dr. Adam Jacobs, division director of family planning at Mount Sinai Medical Center. He expects most women, like Mifflin, would notice that the colors are out of order.

Pfizer said the recall includes 14 lots of Lo/Ovral-28 tablets and 14 lots of the generic version, which is called norgestrel and ethinyl estradiol. Both products are manufactured by Pfizer and sold in the U.S. by Akrimax Rx Products. A lot is a product batch or production run, made at one time and place and can include tens of thousands of individual packages, each with the same identification code for tracking.

Pfizer said the packets are pink with the drug’s brand name or generic name on it, along with the Akrimax name. Pfizer’s logo does not appear on it.