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Taking a single course of a certain type of antibiotics gives rise to high levels of antibiotic resistant bacteria in the mouth, an effect that persists for at least half a year, a new study released Thursday reported.

The extraordinary persistence of the effect startled the scientists who discovered it and others in the field as well — and underscores the need for judicious use of these precious drugs, experts said.

Senior author Dr. Herman Goossens said he and his co-authors assumed that if they followed the subjects in their study for six months they would see the rates of resistant bacteria in participants' mouths return to normal levels. But that didn't happen.

“We were pretty staggered by these data,” said Dr. Goossens, a microbiologist at the University of Antwerp, in Belgium. “We never expected this.”

The finding, reported in the journal The Lancet, is a sharp reminder of the power of antibiotics, suggested Eric Brown, a biochemist at McMaster University in Hamilton.

“So a quick course of antibiotics and a half a year later, you're still carrying resistant organisms. That's a little bit terrifying,” said Dr. Brown, whose laboratory is working on alternative ways to kill bacteria, because of the rising problem of antibiotic resistance.

It also suggests doctors treating patients for bacterial infections should carefully consider which antibiotics they prescribe if those patients have taken antibiotics within the last year.

“If you're a doc who's about to treat a patient who has been treated before, it should have an impact on the decisions you make about what to give that patient,” Dr. Brown said.

The study, which was partially funded by drug maker Abbott Laboratories, is the first to definitively show that antibiotic use is the major factor in the emergence of antibiotic resistance, Dr. Goossens said.

It seems a bit like proving the assumed. Plenty of research has shown that as antibiotic use rises in a population, the rate of antibiotic-resistant infections within that population rises as well. And infection control experts have been campaigning for years to get doctors to cut back on antibiotic use out of a fear that resistance is increasing at such a rate as to threaten the continued usefulness of these drugs.

But those studies could only draw a link between antibiotics and antibiotic resistance — they couldn't be used as proof that use of the drugs causes resistance.

This study proves the effect, Dr. Goossens said, by studying individuals.

Healthy volunteers who took one of two antibiotics from a class called macrolides had a rapid and dramatic spike in antibiotic-resistant streptococci in their mouths. Swabs taken at 28 days, 42 days and 180 days after treatment — which was three days for one antibiotic and seven days for another — showed more than the majority of streptococci in their mouths were resistant to macrolides even at 180 days.

“It . . . should serve as a wake up call for individual prescribing physicians, nurse practitioners, midwives, dentists and others that inappropriate use of antibiotics does have consequences,” said Dr. John Conly, former chair of the Canadian Committee on Antibiotic Resistance and head of the department of medicine at Foothills Medical Centre in Calgary.

“You can liken it to using too many vehicles to get to work and greenhouse gases.”