Nearly half of U.S. adults with high blood pressure had their blood pressure under control by the end of 2010 -- a significant increase from the start of the decade, researchers reported in the American Heart Association journal Circulation.

Improvements in blood pressure control were most likely due to wider use of multiple drug combinations , researchers said.

Researchers at the National Center for Health Statistics (NCHS) interviewed 9,320 hypertensive participants in the National Health and Nutrition Examination Survey (NHANES) 2001-10. By the end of 2010, 47 percent had controlled blood pressure -- up from 29 percent 10 years earlier.

The in-person survey is the first to compare blood pressure control rates before and after the Joint National Committee (JNC7) treatment guidelines were published in 2003.

Under JNC7 guidelines, many patients may need combination therapy with two or more drugs to achieve adequate blood pressure control.

In the study, almost two-thirds of those on combination therapy had controlled blood pressure by 2010, and the use of multiple drugs increased from 37 percent in 2001 to 48 percent by 2010.

Compared with using one drug, single and multiple-pill combinations were associated with 55 percent and 26 percent increased likelihood of control, respectively.

"Much progress has been made in blood pressure control over the last 10-year period and the use of multiple drug combinations apparently has had an effect," said Qiuping Gu, M.D., Ph.D., an epidemiologist at the NCHS.

Lower cost of medications and their availability in generic form as well as increased awareness of the risk of uncontrolled high blood pressure has also had a positive effect.

But some issues continue to be problematic, researchers said.

The national hypertension treatment guidelines recommended thiazide diuretics as initial drug therapy for most patients with uncomplicated hypertension, yet their overall use remains comparatively low. In addition, "nearly half of the hypertensive population is not being treated with combination therapy," said Charles F. Dillon, M.D., Ph.D., co-author of the study.

Moreover, rates were lower for older Americans, African-Americans and people with diabetes and chronic kidney disease. Mexican-Americans were least likely to take any kind of blood pressure medication.

"While there are possibly several factors involved, more needs to be learned about why only 34 percent of Mexican-Americans with hypertension have their blood pressure under control," Gu said. Participants were only asked about medications used in the prior month, so those who might have taken medications previously were classified as non-users.

Furthermore, NHANES blood pressure measurements were only collected one time, so some people in the study may have been misclassified.

Other co-authors are: Vicki L. Burt, Sc.M., R.N.; Charles F. Dillon, M.D., Ph.D.; and Sarah Yoon, Ph.D. Author disclosures are on the manuscript.

NCHS is part of the U.S. Centers for Disease Control and Prevention.

For high blood pressure information visit www.heart.org/hbp .