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Doctors who are overworked, have been trained in other countries or who have been practising longer are more likely to prescribe antibiotics inappropriately, according to new Canadian research that highlights a major problem facing public-health officials.

The study, published today in the Canadian Medical Association Journal, assessed the prescribing habits of hundreds of Quebec doctors over an eight-year period, identifying those who prescribe antibiotics in ways that can lead to drug resistance.

With more illness-causing bacteria growing immune to treatments, and fewer new bacteria-fighting drugs being developed, public-health officials fear that one day there will be no effective way to halt the spread of infectious diseases such as tuberculosis.

"At some point, we will run out of therapies," said Genevieve Cadieux, the study's co-author and a researcher at McGill University's department of epidemiology and biostatistics. "The most daunting concern is that we're not going to have effective drugs to treat illnesses."

The researchers used old records to assess the prescribing habits of 852 doctors in Quebec, beginning in 1990. The researchers looked at how the doctors prescribed drugs to about 170,000 patients diagnosed with either a bacterial or viral infection.

Two major prescribing habits were classified as inappropriate: The use of antibiotics to treat viral infections, and the overuse of broad-spectrum antibiotics to treat bacterial infections.

Both of those practices have been shown to promote drug resistance and increase the likelihood of drug-related side-effects. Prescribing drugs unnecessarily also adds a cost burden to the health-care system.

The researchers, who were looking for predictors of inappropriate antibiotic prescribing, found three main risk groups.

First, doctors with high-volume practices made more prescription errors compared to those with low-volume practices.

Second, international medical graduates were more likely than University of Montreal graduates to prescribe antibiotics for viral respiratory infections.

And third, inappropriate prescribing increased with a doctor's time in practice.

"The issue is, what is really driving that?" said Robyn Tamblyn, another McGill researcher who co-authored the report. She said more research is needed to determine why doctors in these broad categories seem to be more likely to prescribe inappropriately.

The results can be applied to other jurisdictions across Canada and the United States, because they have similar accreditation standards to those used in Quebec, she said.

Michael Gardam, director of infection prevention and control at Toronto's University Health Network, said the study's findings didn't surprise him.

During his hospital rounds, he says, he often comes across patients who have been prescribed antibiotics they don't need. He says it's more common for him to take patients off antibiotic treatments they've been previously prescribed than to put them on the drugs.

"A lot of physicians look at antibiotics as a benign intervention," Dr. Gardam said. "We need to start looking at them as a double-edged sword."