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Doctors in the United States routinely prescribe potentially harmful drugs to older patients, and the problem is particularly acute in the South, a new study shows.

The analysis found that more than one in five seniors on Medicare in the South were prescribed medications that health authorities have specifically advised doctors to avoid giving to older patients because of their severe side effects. Compared with people 65 and older living in New England, those living in the southern region from Texas to South Carolina were about 12 percent more likely to be prescribed a high-risk medication.

The researchers suspected that factors like education, socioeconomic status and access to quality medical care might be driving some of the regional differences. And to some extent, that appeared to be the case. As socioeconomic status grew lower, for example, the likelihood of being prescribed a high-risk drug increased. But even after accounting for these factors, the researchers found that the disparity persisted.

“We can’t specifically identify the reason the southern states have these rates that are so much higher,” said Dr. Amal Trivedi, an author of the study and an assistant professor of health services, policy and practice at Brown University’s Alpert Medical School. “But I think it’s important for physicians and patients to be aware of it.”

Dr. Trivedi and his colleagues published their findings in the latest issue of The Journal of General Internal Medicine. For the study, they referred to a list of 110 drugs to avoid in the elderly, compiled by the National Committee for Quality Assurance. Many of the drugs are widely used, often with few or moderate side effects in younger patients, but their risks are magnified in the elderly.

On the list are anti-anxiety medications like Valium. The drug, a benzodiazepine, can be harder for elderly patients to metabolize, resulting in the drug staying in their systems for longer periods of time. That can lead to prolonged sedation, and in turn potentially deadly falls and fractures. Because of their side effects and potential to cause addiction, benzodiazepines are not usually recommended for the elderly. But when used as a last resort, there are relatively safer, short-acting alternatives, said the lead author of the study, Danya Qato, a pharmacist and doctoral candidate in health services research at Brown.

Several muscle relaxants and diabetes medications can also remain in elderly patients’ systems for longer periods, causing a higher rate of complications.

“We started this study because we know that these medications are likely to have more harms than benefits in older patients,” Dr. Trivedi said. “We have tried to reduce the use of these medications, and it’s important to figure out exactly how common they are among the elderly and what types of factors contribute to their use.”

The researchers looked at data on more than six million older men and women from across the country who were enrolled in Medicare Advantage plans. Over all, they found that 1.3 million of those seniors, or roughly one in five, had been prescribed at least one high-risk medication in 2009 even though many of the drugs had safer substitutes. About 5 percent of the seniors in the study had been prescribed at least two medications from the list.

The city with the most alarming rate was Albany, Ga., where nearly 40 percent of seniors on Medicare had received a prescription for a high-risk drug. The city with the highest rate of seniors receiving at least two high-risk prescriptions was Alexandria, La.

The simultaneous use of multiple medications, a phenomenon known as polypharmacy, is a growing problem among seniors. According to other research, the average person over 65 takes at least four prescription drugs — a practice that can lead to dangerous and unexpected interactions. Adverse effects brought on by the combination of multiple drugs are thought to be responsible for nearly a third of all hospital admissions.

“Polypharmacy is a major public health problem,” Dr. Trivedi said. “Sometimes less is more, particularly for the elderly.”

Ms. Qato said it was unclear why doctors in the South were more prone to prescribing the blacklisted drugs to their older patients and it could be that patients were simply more likely to ask for them. Either way, she said, doctors and patients should be aware of them.

“These lists are readily available on the Internet,” she said. “The take-home message for patients is to take ownership of your medications and to regularly review them with your pharmacists or physicians.”