PRESIDENT OBAMA, backing the decision to prevent the Plan B emergency contraceptive from being sold to girls under 17 without a prescription, emphasized the need to “apply some common sense to various rules when it comes to over-the-counter medicine.” We agree with that standard, and we understand Mr. Obama’s instinctive reaction, as the father of two daughters, that parents should be involved before girls resort to emergency contraception. But the Obama administration ultimately made the wrong call on this difficult and emotionally charged issue.

Science argues in favor of making Plan B available without a prescription, but science cannot provide the full answer. The decision to broaden access to Plan B presents difficult issues of policy, taking into account concern about parental rights, the unfortunate fact of widespread sexual activity among teenagers and the importance of ensuring access to effective contraception. In the end, this balance tilts, in our judgment, in favor of broader availability of Plan B.

The medication has been available without a prescription to women 17 and older since 2007. Earlier this year, its manufacturer, Teva Women’s Health Inc., sought approval from the Food and Drug Administration to make the drug available over the counter to all ages. The agency agreed, finding that Plan B was, as FDA Commissioner Margaret Hamburg maintained, “safe and effective in adolescent females” and could be used properly without expert advice.

Health and Human Services Secretary Kathleen Sebelius took the extraordinary step of overruling the FDA, saying there was not enough evidence that younger teens and preteens could “use the product appropriately.” Mr. Obama said the secretary felt a 10- or 11-year-old should not be able to buy, “alongside bubble gum or batteries . . . a medication that potentially, if not used properly, could end up having an adverse effect.”

This recitation omits some important facts. Plan B has been found safe and effective for younger teens; otherwise it would not be available to them by prescription. The 10-year-olds about whom Mr. Obama worries can buy other medications over the counter that risk even more severe side effects than the nausea or vomiting associated with Plan B. In any event, pregnancy, especially for young girls, is far riskier. The cost of the drug is about $50, meaning that Plan B is not apt to be an impulse buy or a recurring purchase in lieu of regular birth control. Moreover, the focus on the youngest potential patients is something of a red herring. Fewer than 1 percent of 11-year-old girls are sexually active, yet almost half of girls have had sex by their 17th birthdays, according to the Guttmacher Institute. The administration’s attention to this tiny subset ignores the inevitable impact of its decision on older teens: a higher number of unintended pregnancies and abortions.

It is obviously preferable for parents to be involved in their children’s health care, whether the children are 11 or 16, but girls engaged in sexual activity are not necessarily inclined to enlist parental help. Time is of the essence when it comes to emergency contraception; the sooner it is used, the more likely it is to work. The requirement that a sexually active 16-year-old obtain a prescription will unavoidably result in delaying her use of emergency contraception, if not preventing it. Indeed, the administration’s decision will decrease access for all women, because the medication will continue to be kept behind the pharmacy counter so that the age of the buyer can be verified. As a result, access will be limited to hours when the pharmacy is open. That does not constitute common sense.