Only about half of Americans age 65 and older with high blood pressure have it under control, and not taking medications as prescribed might help explain why.

One in four Medicare beneficiaries — about 5 million — are not following doctor’s orders for taking their blood pressure medications, according to a new Vital Signs report from the Centers for Disease Control and Prevention. High blood pressure affects 70 percent of older Americans and increases the risk for heart disease, stroke, kidney disease and death.

The report, released Tuesday, looked at 2014 data from almost 19 million older Americans enrolled in the Medicare Part D subsidy program that helps beneficiaries pay for prescription drugs.

In a phone call with reporters, CDC director Tom Frieden, M.D., said uncontrolled high blood pressure is the main preventable cause of stroke and may lead to cognitive problems later in life, but is “very poorly treated” in the United States.

State and federal healthcare systems should improve their relationships to encourage people with high blood pressure to take their medications, Frieden said.

“Everyone has a stake in this game,” said Frieden, adding that when doctors, nurses, pharmacists and others do a better job helping patients take their medications correctly, they can “save millions of lives.”

American Heart Association President Steven Houser, Ph.D., said in a statement that the new report “confirms what we have known for some time: while access and affordability have an impact on whether Americans take their high blood pressure medications, there are many other reasons that patients don’t take their prescriptions.”

The findings were “particularly disturbing,” Houser said, because high blood pressure is a leading risk factor for heart disease and stroke, the No. 1 and No. 5 killers of Americans.

Cost of medicines, side effects and complex medication cocktails are among the reasons high blood pressure patients don’t take their medicines, said Frieden.

The statistics showed American Indians, blacks and Hispanics were significantly less likely than their white peers to follow doctor’s orders. Disparities were also seen by region, with beneficiaries living in the South, Puerto Rico and U.S. Virgin Islands being least likely to take their medicines as directed and those in the Midwest being most likely to take them.

“As the report highlights, the variety and complexity of barriers — not to mention the potential combination of these barriers — means no single solution will close this adherence gap,” said Houser, senior associate dean of research at Temple University in Philadelphia. “Policies should be put in place that tackle established reasons for nonadherence, while taking into account the racial/ethnic groups, states and geographic variations identified in the report.”

The CDC report was released a day before the start of the AHA’s Council on Hypertension Scientific Sessions, a conference of international researchers who investigate the relationship between high blood pressure and heart disease, stroke and kidney disease.