TRENTON, N.J. (AP) – Everyone knows condoms prevent pregnancy and protect against sexually transmitted diseases. But how well do they work?

That question is at the center of a debate over whether the labels on condom packages should be changed.

On one side are abstinence advocates, including a conservative Congress member who is blocking appointment of a new federal drug agency chief until the labels are changed. On the other side are “safe sex” advocates who fear label changes could undermine confidence in condoms and increase the spread of AIDS and other sexually transmitted diseases.

Each side has some truth in its argument: Condoms are very effective against the AIDS virus, but data for their effectiveness against some other STDs is surprisingly spotty.

“They do not provide 100-percent protection, but for people who are sexually active they are the best and the only method we have for preventing these diseases,” said Heather Boonstra, a public policy official with the Alan Guttmacher Institute, a non-profit group that researches reproductive health issues.

Boonstra said Republican Sen. Tom Coburn, a physician from Oklahoma, and the abstinence-promoting Medical Institute for Sexual Health are “manipulating this data to drive home their own anti-condom, anti-contraceptive message.”

James Trussell, who serves on the board of the Guttmacher Institute and is director of Princeton University’s Office of Population Research, said there is “absolutely incontrovertible evidence” that condoms reduce transmission of the most serious sexually transmitted disease, AIDS.

“To my mind, everything else is gravy,” Trussell said. “All of this is ideologically motivated. What they’re really concerned about is people who are not married having sex.”

But John Hart, spokesperson for Coburn, said the senator’s June 15 hold on Lester Crawford’s nomination as commissioner of the Food and Drug Administration is an effort to make Crawford obey a 2000 law Coburn sponsored. It requires the FDA to change condom labels to give more information on their “effectiveness or lack of effectiveness in preventing STDs.”

Hart said FDA officials recently have said they will have a draft of the language soon. FDA spokesperson Julie Zawisza said she could not discuss policy issues.

Dr. Marie Savard, a women’s health specialist in Philadelphia, said she has qualms about using the word “ineffective,” but agreed people need reliable information.

“The labeling should be changed to something like, ‘condoms protect better against some STDs than others,’” Savard said.

Currently, FDA requires condom boxes and packets to state: “If used properly, latex condoms will help to reduce the risk of transmission of HIV infection (AIDS) and many other sexually transmitted diseases.” Many brands also state condoms are highly effective in preventing pregnancy.

When latex condoms are used every time and put on early enough, they reduce chances of pregnancy over a one-year period to 3 percent, compared with 85 percent without birth control. Likewise, condoms cut risk of HIV infection by about 80 percent, to less than a 1-percent chance of infection per year.

According to the National Institutes of Health, condoms are impervious to the smallest viruses and only break or slip off 1 percent to 2 percent of the time. But surveys show most people don’t use them properly or consistently, and roughly 12 million Americans each year contract an STD.

A 2001 NIH expert panel, convened at Coburn’s request, examined dozens of published studies. It reported that for STDs besides AIDS and gonorrhea, for which condoms cut transmission by 50 percent to 100 percent, the evidence on protection is unclear because of weak and contradictory studies. Individual studies cited in the report give prevention rates ranging from 18 percent to 92 percent, depending on the disease.