THURSDAY, Oct. 24 (HealthDay News) -- People who take certain commonly used blood pressure medications have a significantly lower risk for Alzheimer's disease than those who don't, a new study suggests.

Although it remains unclear exactly how drugs such as ACE inhibitors or diuretics might protect the brain, researchers say these new findings could lead to a better understanding of Alzheimer's and new treatments to slow or delay the progression of the memory-robbing disease.

"We found a risk reduction by 50 percent. That tells you there must be something there," said study leader Dr. Sevil Yasar, an assistant professor of medicine in the department of geriatric medicine and gerontology at the Johns Hopkins University School of Medicine.

The study involved information compiled from more than 2,200 older adults between 75 and 96 years of age. They had originally enrolled in an observational study examining whether the herb ginkgo biloba could reduce the risk of Alzheimer's.

The answer to that question was no, but the researchers were able to use the data already collected to conduct a separate analysis on the protective effect of some commonly prescribed blood pressure drugs, including diuretics, ARBs and ACE inhibitors.

The study, published recently in the journal Neurology, showed that regular use of these medications cut the risk of Alzheimer's dementia by at least half.

These drugs are used by millions of older Americans. For example, Lasix is one of the most commonly prescribed diuretics. Examples of ACE inhibitors include Lotensin, Capoten and Vasotec. Teveten and Avapro are two commonly used ARBs.

The researchers said diuretics, which are the first-line treatment for high blood pressure, also were linked to a 50 percent lower risk of Alzheimer's disease among participants who were already showing signs of "mild cognitive impairment" -- the slight impairment in thinking and memory that is often a precursor to Alzheimer's.

Exactly how these drugs reduced the risk for Alzheimer's dementia, however, is still unclear. One theory is that the protective effect is the result of lower blood pressure.

High blood pressure is a known risk factor for long-term thinking and memory problems, said Dr. Matthew McCoyd, assistant professor of neurology at Loyola University Medical Center in Chicago.

"High blood pressure increases the risk of small vessel ischemic disease, in which the small blood vessels in the brain get smaller and tighter," said McCoyd, who was not involved in the study. "This can lead to a number of problems with [thinking and memory]." A reduction in blood pressure can reduce injury to the part of the brain involved with memory, he said.

Still, if reductions in blood pressure were at the root of the drugs' benefit, then all blood pressure medications should have lowered patients' risks for Alzheimer's disease. But not all classes of these drugs had a protective effect, according to the research.

"We did not find any beneficial effect from calcium channel blockers, which was surprising and disappointing," Yasar said. Beta-blockers also were not associated with a reduced risk for Alzheimer's dementia, she said.

Prior studies have suggested that a specific subgroup of calcium channel blockers might still have a protective effect. "It's not a finished story," Yasar said. "There is something to it, but a larger sample size is needed for an analysis of this subgroup."

She also said the study was limited by the fact that the data was collected to assess the effects of ginkgo biloba. As a result, the researchers were not able to determine if the patients took their blood pressure medication as prescribed or if they had used these drugs in the past.

So the question remains: Do certain blood pressure medications reduce the risk for Alzheimer's dementia due to a lowering of blood pressure, or is something else going on?

"The inherent overall health benefit of these medications isn't as clear independent of high blood pressure," McCoyd said. "Do these drugs provide additional benefits in terms of brain health? If that's the case, patients at high risk for brain disease, particularly dementia, may benefit from repurposing these medications."

Blood pressure drugs are already used to treat other conditions independent of blood pressure, such as tremor and headache, McCoyd said. "With so many people affected by [thinking and memory] changes, using these drugs to prevent or delay dementia would probably have the greatest social impact."

McCoyd cautioned, however, that a lot more research is needed before people with normal blood pressure would be automatically prescribed blood pressure drugs to decrease their dementia risk. Although these drugs are widely considered safe, all drugs have side effects, he said. Common side effects of blood pressure medications include dizziness, cough, rash, fatigue, nausea and headache.

More observational studies like this one -- which cannot prove a direct cause-and-effect link -- are not the answer, Yasar said.

"It's time to do clinical trials for these [blood pressure] drugs," she said. "We know these medications lower blood pressure, but could there be something else going on? Our hypothesis is that yes, there is something else."

More information

The U.S. National Institute on Aging provides more information on Alzheimer's disease prevention strategies.