A high blood pressure drug has the potential to slow down Alzheimer's disease by improving the flow of blood to parts of the brain linked to memory, a study suggests.

Researchers at Radboud University in the Netherlands found the drug nilvadipine increased blood flow to the brain's memory and learning centre among people with Alzheimer's without affecting other parts of the brain.

The findings, published in the journal Hypertension, indicate that the known decrease in cerebral blood flow in patients with Alzheimer's can be reversed in some regions.

However, an important question is whether this observed increase in cerebral blood flow translates to clinical benefits, the researchers said.

Alzheimer's disease is the most common form of dementia.

The risk for the disease increases with age and the causes are largely unknown.

Previous research has shown that blood flow to the brain declines in early Alzheimer's disease.

Nilvadipine is a calcium channel blocker used to treat high blood pressure.

Researchers sought to discover whether nilvadipine could help treat Alzheimer's disease by comparing the use of nilvadipine and a placebo among people with mild to moderate Alzheimer's disease.

They randomly assigned 44 participants to receive either nilvadipine or a placebo for six months.

Neither researchers nor the participants knew who received the drug or the placebo that was evenly divided among the two groups.

At the study's start and after six months, researchers measured blood flow to specific regions of the brain using a unique magnetic resonance imaging (MRI) technique.

Results showed that blood flow to the hippocampus -- the brain's memory and learning centre -- increased by 20 per cent among the nilvadipine group compared to the placebo group.

Blood flow to other regions of the brain was unchanged in both groups.

"This high blood pressure treatment holds promise as it doesn't appear to decrease blood flow to the brain, which could cause more harm than benefit," said Jurgen Claassen, an associate professor at Radboud University Medical Center.

"Even though no medical treatment is without risk, getting treatment for high blood pressure could be important to maintain brain health in patients with Alzheimer's disease," Claassen said.

Researchers note that sample sizes were too small and follow-up time too short to reliably study the effects of this cerebral blood flow increase on structural brain measures and cognitive measures.

Study participants were screened between 2013 and 2015 as part of a larger research project comparing nilvadipine to placebo among more than 500 people with mild to moderate Alzheimer's disease.

In that larger project, effects on cerebral blood flow were not measured. Overall, no clinical benefit was noted with use of nilvadipine.

However, a subgroup of patients with only mild symptoms of disease did show benefit, in the sense of a slower decline in memory.

Previous studies have hinted that high blood pressure treatment could reduce the risk of developing dementia.

The researchers think that beneficial effects on brain blood flow could explain part of this effect.