The researchers analyzed data from the Cardiovascular Health Study, a long-term study of cardiovascular risk factors that involved 5,888 people over 65 years old. The mean age of participants was 75 years old and most participants were women.

The drugs that researchers believe are protective are part of a class known as ACE inhibitors -- specifically those types that reach the brain and may help reduce the inflammation that might contribute to Alzheimer's disease.

"For older adults who are going to take an ACE inhibitor drug for blood pressure control, it makes sense for their doctors to prescribe one that goes into the brain," said Kaycee Sink, lead researcher and an assistant professor of gerontology.

Some ACE (angiotensin-converting enzyme) inhibitors are known as centrally acting because they can cross the blood brain barrier, a specialized system of tiny blood vessels that protects the brain from harmful substances in the blood stream. Centrally acting drugs include captropril, fosinopril, lisinopril, perindopril, ramipril and trandolapril.

The study found a link between taking centrally active ACE inhibitors and lower rates of mental decline. For each year that participants were exposed to ACE inhibitors that cross the blood brain barrier, the decline in test results was 50 percent lower than the decline in people taking other kinds of high blood pressure pills.