Antibiotics can lead to bacterial drug resistance in a patient up to a year after treatment, according to a study published in the British Medical Journal Wednesday.

The literature review of 24 earlier studies found bacteria's resistance to antibiotics was strongest in the month following treatment, but could last up to 12 months afterward.

"This goes on for quite a lot longer than we had thought," Dr. Alastair Hay, lead author of the study and a physician and lecturer in health care at the NIHR National School for Primary Care Research in Bristol, UK, told DOTmed News.

The study was the first major review of the literature known to the researchers to check how long after therapy drug resistance in patients lasts, Hay said.

The review looked over earlier studies to find the effects on an individual's bacterial drug resistance after treatment with several common antibiotics in the respiratory tract, urinary tract; and MRSA infections. It found that the greater number of courses before treatment, the greater the likelihood researchers would extract bacteria in the patients resistant to the antibiotics.

For two of the most commonly prescribed first-line antibiotics, amoxicillin and trimethoprim, the researchers said they found evidence of a dose-response relationship, meaning the greater the dose, the greater the subsequent resistance.

Resistance happens as antibiotic exposure kills off susceptible bacteria, artificially creating a favorable environment for resistant bacteria to flourish, Hay said. But the state isn't stable. In a randomized, controlled study Hay said was among the best-designed of the ones looked at, resistance was highest in the week after treatment but declined over time. The odds ratio for resistance fell from around 12 after the first week, to around 6 at one month, and 2.2 after half a year.

"Being a resistant bacteria is harder work than being a susceptible one," he said. "If there's no other environmental pressure, the bacteria will slowly revert back to being susceptible. It's their most energy-efficient state."

RESISTANCE COSTS

This antibiotic resistance has serious costs. In an editorial accompanying Hay's article, researchers from Duke University in North Carolina and Uppsala University in Sweden point out treating hospital-acquired infections derived from just six bacteria cost the United States around $1.9 billion a year, more than the costs for treating the flu. Costs from the European Union add another nearly $1.9 billion to the global total.

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