WASHINGTON (AP) – The most-used medicines in Medicare’s prescription drug program are generics, but the program spends the most on brand-name drugs, led by the heartburn treatment Nexium, according to an unprecedented release of government data on Thursday.

That contrast sheds light on prescribing practices and how they might be used to save money, specialists say.

More than a million health care providers prescribed $103 billion worth of medications under the popular Part D drug benefit for seniors in 2013, the Centers for Medicare and Medicaid Services said.

Of more than 3,000 drugs prescribed that year, AstraZeneca’s Nexium alone accounted for $2.5 billion of the spending, prescribed to nearly 1.5 million Medicare beneficiaries. GlaxoSmithKline’s asthma drug Advair Diskus accounted for $2.3 billion of the Part D program, followed by AstraZeneca’s cholesterol blockbuster Crestor at $2.2 billion.

Contrast that with the 10 most-prescribed drugs for Part D beneficiaries that year, generics given to many times more patients but costing far less – from $145 million to $911 million for each. Six of the most-prescribed drugs were related to heart disease risks such as blood pressure and cholesterol.

Specialists highlight the contrast as an example of why the data, released publicly for the first time, matter.

Consider: the sixth-most prescribed Part D drug in 2013 was a close cousin of Nexium, a heartburn drug called omeprazole, given to nearly 6.4 million beneficiaries, noted Dr. Michael Steinman, a geriatrician and professor at the University of California, San Francisco. That’s more than four times as many patients as received Nexium, yet total spending on omeprazole was just $643 million.

Omeprazole is a generic version of a Nexium precursor, not Nexium itself.

Still, “from a practical perspective, there is no substantive advantage to giving someone Nexium over omeprazole,” said Steinman, who researches prescribing for older adults. “The main difference between them is cost, marketing, frankly.”

He called the database “a tremendous opportunity for identifying how doctors are prescribing medications in the U.S. and finding places where we can be doing better.”

When the government agency mapped generic use, it found doctors in the West and Midwest prescribed cheaper generics for seniors far more than physicians in the rest of the country. Generic prescribing was lowest in parts of the South and along the East Coast.

Deputy Administrator Sean Cavanaugh said the government was releasing the data for transparency. While Medicare officials have long analyzed such data in setting policy, “there are many, many smart minds in this country” that might uncover new insights from it, he said.

The government has been attempting to steer providers toward higher-quality, more cost-effective medicine in part by releasing data on payments to health care providers. Last year, Medicare opened its huge claims database, showing program payments to more than 825,000 providers for 2012, as well as data detailing drug industry payments to physicians, such as research grants and travel junkets.

The Part D database identifies doctors by name, allowing searches of what they prescribe. But the Medicare agency cautioned against drawing too many conclusions from such individual data, noting that sometimes a large practice’s prescriptions are recorded under one partner’s name.

The American Medical Association echoed those cautions.

“We are also troubled by the lack of context provided with the data that could help explain physician prescribing practices and pharmacy filling practices before conclusions are drawn,” AMA President Dr. Robert M. Wah said in a statement.

But researchers will be interested in checking how often doctors in particular areas prescribe drugs known to be risky for seniors, said Dr. Adriane Fugh-Berman of Georgetown University, a critic of doctor-drug industry relationships.

For example, the antipsychotic Abilify accounted for $2.1 billion in the Part D spending in 2013, an amount Fugh-Berman found high. Antipsychotics are approved to treat such disorders as schizophrenia. But specialists repeatedly warn they’re used too often to calm seniors with dementia – despite Food and Drug Administration warnings that they can increase those patients’ risk of death.

About 36 million people, or about two-thirds of Medicare beneficiaries, are enrolled in the Part D program, which is delivered through private insurance plans.