Curbing the routine use of preventive antibiotics before dental work may have contributed to a rise in heart valve infections in England, a new study suggests. In the U.S., the highest risk patients still get these drugs and no similar trend has been seen.

Mouths are full of bacteria, and certain dental procedures can let them enter the bloodstream, travel to the heart and cause a serious infection called endocarditis, which proves fatal up to 10 to 20 percent of the time. People with artificial heart valves and other implants are at high risk for this, and people with leaky natural heart valves face a moderate risk.

It used to be routine to give those people antibiotics — usually a single penicillin pill — just before dental work. But there was little evidence the preventive treatment lowered infection rates and the drugs sometimes cause serious allergic reactions. Overuse of antibiotics also promotes drug-resistant germs — a public health concern.

So in 2007, the American Heart Association and others said the drugs should only be used for the highest risk patients; a year later, regulators in England recommended stopping them for all patients.

Researchers from the University of Surrey and Oxford University did a study to see what happened after that in England.

They found that prescriptions for preventive antibiotics fell from an average of 10,900 per month in the four years before the policy change to 2,236 per month in the five years after it.

Starting in March 2008, heart valve infections started to rise above usual levels, according to hospital records. Five years later, "there were approximately 35 extra endocarditis cases per month than would have been expected" if trends before the antibiotic prescribing change had continued, said one study leader, Dr. Martin Thornhill of the Sheffield School of Clinical Dentistry in England.

The work doesn't prove fewer antibiotics caused the surge, he and others stressed. Heart infections have been rising everywhere because other procedures that make them more likely, such as implanted medical devices and kidney dialysis, have become more common.

There also was never much proof that the preventive antibiotics lowered endocarditis risk. One study even found that brushing teeth twice a day was five times more likely to push bacteria into the bloodstream than having a tooth removed, said Dr. Dhruv Kazi of San Francisco General Hospital, an expert with no role in the study.

"These should not prompt changes in prescription practices," he said of the new results.

Dr. Ann Bolger, a cardiologist at the University of California, San Francisco, who worked on the Heart Association policy, agreed.

"We don't want people to be alarmed. In the U.S., people who are most at risk are still getting them," she said of the drugs. It wouldn't be wise "to give up on the policy too soon and return to a treatment with no benefit and known risks."

The study was discussed Tuesday at a Heart Association conference in Chicago and published by the British journal Lancet. The work was sponsored by the U.S. National Institutes of Health, the British charity group Heart Research UK, and Simplyhealth, a British insurer.