We all know that every day, people in Mexico come across the border in pursuit of something they can't find in their country. What you may not know is that every day, people in the United States go across the border to Mexico for the same reason. They aren't looking for jobs. They're looking for birth control pills.

Why would that be? After all, Americans can get the pill without leaving their country. The reason is that in Mexico, they don't need a prescription. They don't have to see a doctor and present a scrip to a pharmacist.

They can just walk into a store and buy what they want. The cross-border alternative is cheaper, and they don't even have to go themselves—a friend or relative can pick them up.

Last week, the Obama administration gave up its transparently political effort to enforce an age restriction on over-the-counter purchases of Plan B, the "morning-after pill" that can prevent pregnancy after sex. This helpful development creates an anomaly: Women and girls will be able to get emergency contraceptives without a prescription—but not the regular oral contraceptives that would make them unnecessary.

It's like saying we'll treat you if you come down with the flu, but we won't give you a flu vaccine so you don't get it in the first place.

There is no obvious explanation why something used safely over half a century by hundreds of millions of women should remain so restricted. In most countries, it's not. But the U.S. government presumes that women are incapable of making this decision without the approval of a doctor.

This outmoded paternalism can't be justified by the health risks of oral contraceptives. Plenty of over-the-counter medicines are considerably more dangerous. "Nonsteroidal medicines kill far more people than birth-control pills," Dr. Eve Espey, a professor of obstetrics and gynecology at the University of New Mexico, referring to painkillers like Advil and Motrin, told The New York Times.

Oral contraceptives are not entirely devoid of danger. They can slightly raise the risk of certain cancers, while lowering the risks of others. They can slightly increase the incidence of heart attack and stroke, particularly for women who are over 35, have high blood pressure or smoke. But they are safe enough that doctors often prescribe them even for patients who are chaste as nuns—to alleviate acne, severe cramps and heavy menstruation.

There happen to be a few risks in not taking the pill, like pregnancy and childbirth. Besides being desperately unwanted in many cases, they harbor serious perils of their own: diabetes, high blood pressure, obesity, stroke, kidney failure and depression, and soaring college costs. Each year, some 700 American women die while giving birth.

Many contraceptives can help avert these outcomes. But few are as safe and reliable as oral contraceptives. Women who take them properly have only a 1 in 100 chance of getting pregnant in a given year—much better odds than for those who rely on condoms or diaphragms.

Most women would love to be able to get birth control pills without the trouble and expense of seeing a physician. A poll this year found that nearly two of every three American women favor the idea. Some 30 percent of those who are now using less effective contraception, or none, said they would probably start taking the pill if they didn't have to get permission.

Doctors are warming to the idea as well. Last year, the American College of Obstetricians and Gynecologists endorsed the change as a way to improve access and stem the tide of unintended pregnancies—which, they noted mournfully, have remained stuck at about half of all pregnancies for two decades. Fewer unwanted pregnancies would also mean fewer abortions.

Whether and when the change will occur is anyone's guess. The FDA typically waits for a pharmaceutical manufacturer to ask for reclassification before it considers action. So far, none has.

But Ibis Reproductive Health, an international nonprofit, has discussed that possibility with some drug makers. "After ACOG issued its opinion last year, the interest among pharmaceutical companies escalated a bit," Britt Wahlin, director of development and communications, told me.

Scraping the existing restriction would be a triumph of logic. Though it may pain Michael Bloomberg, a woman currently needs no prescription to have sex. She can do it strictly on her own choice. Now, there's a concept.