Elderly Americans are prescribed medications in inexplicably different ways depending on where they live, according to a new report from Dartmouth researchers.

The most depressed older patients—or at least the ones being medicated -- live in parts of Louisiana and Florida. There’s a cluster with dementia around Miami. And the seniors who have the most trouble sleeping? They live, perhaps unsurprisingly, in Manhattan.

The study by the Dartmouth Institute for Health Policy and Clinical Practice examined geographic variations in the drugs elderly Medicare patients received in 2010. Researchers mapped where patients got medications they clearly needed and where they got drugs deemed risky for the elderly. They also looked at differences in the use of so-called discretionary drugs, which they say are widely prescribed but of uncertain benefits.

The report’s findings underscore those of a ProPublica investigation in May, which found that some doctors who treat Medicare patients often prescribe drugs that are dangerous or inappropriate for certain patients. ProPublica also found that the federal officials who run Medicare have done little to scrutinize prescribing patterns in their drug program, known as Part D, or question doctors whose practices differ from their peers.

Officials from the Centers for Medicare and Medicaid Services could not be reached to answer questions about the study. They have previously said that the primary responsibility for overseeing prescribing belongs to private insurers that administer the program. Still, they have acknowledged that Medicare should and will do more to track prescribing in Part D and follow up on unusual patterns.

The Dartmouth researchers did not look at the habits of individual doctors, as ProPublica did, but instead looked at the percent of patients in each region who received certain types of medications. Regionals boundaries were based on where patients would be referred for hospital care.

For example, 17 percent of elderly patients in Miami received a prescription for a dementia drug in 2010, while less than 4 percent of patients in Rochester, Minn., and Grand Junction, Colo., got one. Nationally, the average was 7 percent, according to the report, titled the Dartmouth Atlas of Medicare Prescription Drug Use.

There were similar differences by location for antidepressants. In Miami, almost one-third of elderly Medicare enrollees received at least one prescription for such drugs and about one-quarter of those in a swath of Louisiana did. In Honolulu, just 7 percent got one.

The report does not address whether the patients had diagnoses that would warrant the use of these medications. It also does not include disabled patients under 65 who are also covered by Part D.

Researchers examined whether patients in different regions had been given widely accepted drug treatments following health emergencies, for instance a beta blocker after a heart attack or an osteoporosis drug after certain fractures.  And they calculated the percentage of seniors who were given drugs labeled risky by the American Geriatrics Society because they are known to affect their cognition and balance.

“We see that some clinicians are not achieving a level of effective medication use” compared to their peers, said Dr. Nancy Morden, a lead author of the report. “Conversely, some clinicians are putting their patients at much higher risk by using hazardous medications at a much higher rate than their peers.”

The report does not tackle two of the most fraught issues in prescribing today: the use of narcotic painkillers and anti-psychotics, especially to treat dementia in the elderly.

Morden said she was surprised to find that, in some regions, large percentages of patients were getting discretionary drugs that were moderately beneficial, like those for acid reflux -- and not getting the ones that could save their lives, like the beta blockers or cholesterol-lowering drugs.

 “What are we doing?” she said. “It’s surprising to me that we can use so much of our energy to pursue medications that give us far less in terms of health. I worry that it’s coming at the cost of getting the effective medications.”

People in some regions of the country are healthier than in others. But Morden said that does not explain the wide variations her group found in so many different categories of drugs. That may be a signal that patients are not being adequately informed about the risks, benefits and costs of the drugs, she said. Doctors also may be unaware of how different their practices are from the peers in other parts of the country.

Overall, researchers found that the elderly in Miami fill more prescriptions than anywhere else. On average Miami area patients got nearly 63 per person, including refills, in 2010, compared to a national average of 49. Seniors in Miami also had the highest average spending on prescriptions that year, $4,738 compared to $2,670 nationally.

In the report, Morden and her co-authors encourage policymakers to seek ways of reducing geographic variation in the way medications are prescribed. They also urge patients to ask their doctors about whether a drug is truly needed for them.

The Dartmouth group has previously examined how costs and use of services in the Medicare program differ markedly across the country. They note that some of the highest-spending regions in terms of drug costs were also among the highest users of other types of medical services.