Adults diagnosed with attention-deficit/hyperactivity disorder ( ADHD) are almost three times more likely to go on to develop dementia, according to a new study that looked at 109,218 adults with and without the condition over a period of 17 years.

The researchers, from institutions in Israel and the US, found that 13.2 percent of the participants with ADHD went on to develop dementia over the course of the study, compared with 7 percent of those without an ADHD diagnosis.

After adjusting for other potential factors (such as heart problems) and calculating a hazard ratio which also considers how quickly dementia occurred, the conclusion was that those with ADHD were 2.77 times more likely to develop dementia conditions, including Alzheimer's disease.

As well as offering scientists new insight into the neurological mechanisms that might trigger dementia, the study also helps to identify more people who could be at greater risk so that precautions can be taken.

"By determining if adults with ADHD are at higher risk for dementia and if medications and/or lifestyle changes can affect risks, the outcomes of this research can be used to better inform caregivers and clinicians," says neurologist Michal Schnaider Beeri from Rutgers University.

More than 3 percent of adults in the US have an ADHD diagnosis. It affects attention, movement, and impulse control, and the researchers suggest the related neurological processes may impact the ability of the brain to protect against cognitive decline later in life.

While the data isn't enough to show ADHD causes dementia directly, it strongly hints at some kind of relationship – and as a result, that it's important to monitor adults for symptoms of the disorder as they get older.

"Symptoms of attention deficit and hyperactivity in old age shouldn't be ignored and should be discussed with physicians," says Stephen Levine, a public health scientist from the School of Public Health at the University of Haifa in Israel.

Treatments for ADHD vary depending on the person and their age, but a combination of medication and behavioral therapy is often used. Here, those with ADHD who also took psychostimulants weren't statistically shown to be at a greater risk of developing dementia later in life.

That suggests it's possible that certain changes to treatments for ADHD could lower the risk of dementia – but more detailed research across a large sample of people is going to be required to know for sure.

"Physicians, clinicians and caregivers who work with older adults should monitor ADHD symptoms and associated medications," says Abraham Reichenberg, a brain and behavior scientist at the Icahn School of Medicine at Mount Sinai.

The research has been published in JAMA Network Open.