President Obama said Thursday that he supports his administration’s decision to block unrestricted sale of the morning-after pill to people younger than 17, a move that dismayed women’s advocates when it was announced by Health and Human Services Secretary Kathleen Sebelius.

Sebelius said Wednesday that she had overruled the Food and Drug Administration, which had decided to make the contraceptive available to people of all ages directly off drugstore and supermarket shelves, without a prescription.

Obama said he did not get involved in the decision to require a prescription for girls 16 and under before it was announced, leaving it up to Sebelius.

But, he said: “I will say this. As the father of two daughters, I think it is important for us to make sure that we apply some common sense to various rules when it comes to over-the-counter medicine.”

“And as I understand it, the reason Kathleen made this decision was she could not be confident that a 10-year-old or an 11-year-old, going to a drug store, should be able to, alongside bubble gum or batteries,” purchase a powerful drug to stop a pregnancy, Obama said. “I think most parents would probably feel the same way.”

Women’s groups have reacted with alarm to the decision, and Obama’s remarks did nothing to tamp down the furor.

“It’s not good enough to say what you want for your own two daughters. The issue here is science and medicine,” Eleanor Smeal, president of the Feminist Majority, said, describing herself as “dumbstruck” and “stunned” by the move.

Food and Drug Administration officials said they could not recall another time that the Department of Health and Human Services had vetoed such a decision.

“There is adequate and reasonable, well-supported, and science-based evidence that Plan B One-Step is safe and effective and should be approved for nonprescription use for all females of child-bearing potential,” FDA Administrator Margaret A. Hamburg said in a statement, revealing a rare public split.

“However, this morning I received a memorandum from the Secretary of Health and Human Services invoking her authority under the Federal Food, Drug, and Cosmetic Act to execute its provisions and stating that she does not agree with the Agency’s decision.”

In a statement and separate letter to Hamburg, Sebelius said she reversed the FDA’s decision because she had concluded that data submitted by the drug’s maker did not “conclusively establish” that Plan B could be used safely by the youngest girls.

“About 10 percent of girls are physically capable of bearing children by 11.1 years of age. It is common knowledge that there are significant cognitive and behavioral differences between older adolescent girls and the youngest girls of reproductive age,” Sebelius said.

Her action means that instead of being able to pick up Plan B off store shelves, like condoms and spermicides, girls 16 and younger still need a doctor’s prescription to obtain it. Women 17 and older can buy the pill without a prescription but must show proof of age to a pharmacist.

The decision shocked and angered the doctors, health advocates, family-planning activists, lawmakers and others who supported relaxing the restrictions to help women, including teenagers, prevent unwanted pregnancies.

“We are outraged that this administration has let politics trump science,” said Kirsten Moore of the Reproductive Health Technologies Project, a Washington-based advocacy group. “This administration is unwilling to stand up to any controversy and do the right thing for women’s health. That’s shameful.”

Susan F. Wood of George Washington University, who resigned from the FDA in 2005 because of delays by the George W. Bush administration in relaxing restrictions on Plan B, said she was “beyond stunned” by the decision.

“There is no rationale that can justify HHS reaching in and overturning the FDA on the decision about this safe and effective contraception,” Wood said. “I never thought I’d see this happen again.”

Opponents of easier access, meanwhile, hailed the decision, saying relaxing the rules would have exposed girls and women to risks from taking high doses of a potent hormone and misusing the medication; interfered with parents’ ability to monitor their children; and made it easier for men to prey on vulnerable minors.

“Plan B can act in a way that can destroy life,” said Jeanne Monahan of the Family Research Council, a conservative advocacy group. “A decision to make Plan B available for girls under the age of 17 without a prescription would not have been in the interest of young women’s health.”

A long controversy

Plan B has long been controversial and was the focus of one of the most contentious health disputes during the Bush administration. It works primarily by preventing an egg from being fertilized. But critics focus on the chance that it might prevent a very early embryo from implanting in the womb, an action they consider equivalent to an abortion. As a result, some doctors refuse to write prescriptions for it, some pharmacists refuse to fill requests, and some hospitals refuse to provide it to patients.

Wednesday’s decision came as the administration is trying to defuse rising tensions with the Catholic Church over several issues, including a proposed mandate that private insurers provide women with contraceptives for free and a federal denial of an anti-human-trafficking grant to the U.S. Conference of Catholic Bishops.

“I welcome the . . . decision not to expand nonprescription use of Plan B to all minors of childbearing age,” said the conference’s Deirdre McQuade. Plan B “could endanger the lives of newly conceived children through its abortifacient action, put minors at risk for unnecessary side effects, undermine parental rights and contribute to higher STD [sexually transmitted disease] rates.”

A senior White House official who spoke on the condition of anonymity said the administration trusted Sebelius on both the substance and politics of the issue. The official noted that Sebelius had been governor of Kansas, a conservative state that required deft decision-making by a Democratic governor.

A Health and Human Services spokesperson said in an e-mail that the “decision was based on a careful consideration of the science.”

Obama pledged in 2009 to prevent politics from interfering with scientific decisions. The Bush administration had been accused of censoring federal scientists on climate change and other hot-button issues.

But Wednesday’s decision was not the first time the Obama administration has overruled the scientific advice of senior officials. In September, Obama pulled back smog standards proposed by Environmental Protection Agency Administrator Lisa P. Jackson, saying they would impose too heavy an economic burden.

The administration has long tried to find common ground on issues related to abortion and birth control. The White House hosted meetings aimed at finding areas of agreement among activists, but the effort foundered. In 2010, the administration tried to appease both sides in the debate over sex education and abstinence, launching a campaign supporting programs of both types.

Rules gradually relaxed

The Plan B request followed a series of steps that have gradually made the emergency contraceptive easier to obtain.

The drug contains a higher dose of a synthetic form of the hormone progesterone than is found in many standard birth-control pills. It was approved in 1999, but only for women who obtained a prescription from a doctor. Taken within 72 hours of unprotected sex, the pill has been shown to be 89 percent effective at safely preventing pregnancy.

In 2003, Plan B’s maker asked the FDA to allow the contraceptive to be sold without a prescription so women would not have to scramble — often in late-night or weekend panics after sex without protection, a failed condom or a rape — to find both a doctor to write a prescription and an open pharmacy.

The FDA delayed a decision on that request for three years, ignoring recommendations by outside advisers and internal reviewers. It approved nonprescription sales in 2006, but only to women age 18 and older.

In 2009, the agency allowed the sale of Plan B One-Step, a new version that works with one pill instead of the original two, to 17-year-olds without a prescription. But the decision came only after a federal judge ordered the change in response to a lawsuit.

In February, the drug’s maker, Teva Pharmaceutical Industries, asked the FDA to drop the remaining restriction, citing two new studies that it said showed that girls as young as 11 could use the product properly and safely.

A hearing is scheduled for Tuesday on a motion to declare the FDA in contempt of court for failing to review earlier its decision to leave age restrictions in place.

Another morning-after pill, Next Choice, the generic version of the original two-pill Plan B that sells for about $35, is available only by prescription.

Staff writers N.C. Aizenman and Juliet Eilperin contributed to this report.