Almost 5 million Medicare enrollees are not taking their blood pressure medication as directed, exposing themselves to the risk of experiencing heart disease, stroke, kidney disease, and in some cases, even death, a new report from the U.S. Centers for Disease Control and Prevention (CDC) has shown.

According to CDC Director Tom Frieden, heart disease and strokes account for about one of every three deaths in the United States — killing nearly 800,000 people every year. The study, published Tuesday in the CDC’s Morbidity and Mortality Weekly Report, analyzed 18.5 million Medicare Part D enrollees in 2014, to find that more than a quarter of these either skipped doses or stopped taking the drugs completely.

“That’s particularly troubling, because other research indicates that up to 25 percent of new prescriptions for blood pressure medicine are never even filled in the first place,” HealthDay News quoted Frieden as saying. “Of those prescribed those regimens, maybe a quarter don't even start them, and now we're finding that another quarter don't continue them.”

According to the CDC, seven out of 10 American adults, aged 65 and older, have high blood pressure — with only a little more than half having kept the condition under control.

The study found disparities of drug adherence for different racial and ethnic groups. More than one-third of blacks, Hispanics and American Indians did not take the medication as prescribed, as opposed to almost one quarter of whites or Asian/Pacific Islanders, exposing the former to a higher risk of heart attack and stroke.

Southern U.S. states, along with Puerto Rico, and the U.S. Virgin Islands saw the highest rate of non-adherence in the nation, the report showed. Lead researcher Matthew Ritchey, an epidemiologist with the CDC’s division for heart disease and stroke prevention, reportedly said, “This is another call to action for this part of the country, knowing that they are typically at higher risk for having strokes and heart attacks.”

“A simple action can avoid potentially deadly consequences: take your blood pressure medicine as prescribed,” Frieden said in a press release. “Health care providers can make treatment easier to help people keep their blood pressure controlled.”

Frieden, who said the results were “troubling,” according to CBS News, added, “There’s a reason hypertension is called the silent killer.”