And younger men and older women are particularly vulnerable, researchers warn.
"Although this phenomenon has been hinted at in the medical literature, it is a bit surprising to me as we should not expect anyone to have uncontrolled blood pressure, especially if they are already taking prescription medications for them," said study author Dr. Aayush Visaria. He's a postdoctoral research fellow at Rutgers Institute for Health, Health Care Policy and Aging Research in New Brunswick, N.J.
Among patients in their 70s, women were 29% more likely than men to have uncontrolled high blood pressure, Visaria and his colleagues found. And women 80 and up had a 63% higher risk.
Among 20- to 49-year-olds, the reverse appears to be true.
Compared to women, the risk for treated but uncontrolled blood pressure was 59% higher among men in their 20s, 70% higher among those in their 30s, and 47% higher for men in their 40s.
All told, nearly half of adults in the United States have high blood pressure, increasing their risk for heart attack, stroke, kidney disease and other health complications, including severe COVID-19, according to the American Heart Association.
Researcher Visaria described his findings as worrying.
"Most of these medications are extensively studied," he noted, "and we do know they lower [blood pressure] quite well."
Still, if blood pressure remains above the level considered high -- 130/80 -- it is considered uncontrolled, Visaria explained.
To learn more about the risk for treated but uncontrolled high blood pressure, his team focused on more than 13,200 men and women age 20 and up.
Between 1999 and 2018, all had been enrolled in an ongoing study of health and nutrition. On average, participants were 57 years old, and 7 in 10 were white. And all were taking prescription high blood pressure medication.
Even so, 35% of women and 33% of men were found to have uncontrolled high blood pressure.
While the risk was roughly the same for men and women between age 50 and 69, a huge gender gap was evident on either end of that period, with risk much higher among younger men and much higher among older women.
"It is difficult to ascertain" why, Visaria said.
One possible explanation is an under-representation of older women in blood pressure studies. The result is that "we don't know whether the medications work as well in older women," Visaria said.
Dr. Gregg Fonarow, director of the Ahmanson-University of California Los Angeles Cardiomyopathy Center, said the differences may reflect a variety of complex factors.
Those, he said, include access to care, clinical decision-making, differences in lifestyle choices, socio-economic background, and health and medication history.
Whatever the explanation, "the key message should be that there are major opportunities to improve control of blood pressure of men and women of all ages," said Fonarow, who was not involved in the study.
Visaria agreed. "The main implications are that older women and younger men with high blood pressure may benefit from more frequent monitoring of their BP, to at least be able to detect increases in blood pressure," he said.
Such vigilance is particularly important among older women, Visaria added, given that older women also are prone to under-treatment.
"We know that women in general have a faster rise in their blood pressure after menopause, [although] the reasons are still not fully known," he noted, adding that younger women tend to have a far lower risk for high blood pressure than men.
As a result, when blood pressure rises in older women, doctors and patients may unintentionally overlook it, since they don't feel any symptoms and their blood pressure has been controlled their whole life, Visaria said.
As a result, many older women may not be getting the meds they need to begin with.
The findings were presented Monday at a virtual meeting of the American Heart Association. Research presented at meetings is typically considered preliminary until published in a peer-reviewed journal.
The American Heart Association has more about high blood pressure.
SOURCES: Aayush Visaria, MD, MPH, postdoctoral research fellow, Rutgers Institute for Health, Health Care Policy and Aging Research, New Brunswick, N.J.; Gregg Fonarow, MD, director, Ahmanson-University of California Los Angeles Cardiomyopathy Center; American Heart Association Hypertension Scientific Sessions 2021, Sept. 27, 2021, online