Adults with ADHD may have an increased risk of developing dementia later in life, a new study from Taiwan finds.

In the study, adults who had been diagnosed with ADHD were more than three times more likely to later be diagnosed with dementia, compared with adults who did not have ADHD, the researchers found.

Between 8 and 12 percent of children worldwide have been diagnosed with ADHD, according to the study. Although some children with ADHD may experience a decline in their symptoms as they grow up, this doesn't happen for everyone, according to the National Institutes of Health. In the U.S., 4 percent of adults have ADHD, the NIH says.

The World Health Organization estimates 47 million people worldwide currently have some form of dementia. [10 Things You Didn't Know About the Brain]

Previous research in Taiwan suggested a link between ADHD and certain mental health disorders including bipolar disorder and depression, according to the study, which was published in June in the Journal of Attention Disorders. And a 2011 study found that older adults with one type of dementia, called Lewy body dementia, were more likely to have had ADHD symptoms as an adult than older adults with another type of dementia, Alzheimer's disease.

In the new study, the researchers used data from Taiwan's National Health Insurance Research Database, which contains data on more than 99 percent of the population of Taiwan. From the database, the researchers focused on 675 adults ages 18 to 54 who were diagnosed with ADHD in the year 2000. The study also included more than 2,000 adults who did not have ADHD.

Over a 10-year period, the adults with ADHD were 3.4 times more likely to be diagnosed with dementia than those without ADHD, the researchers found.

The researchers didn't look into the possible causes that could be behind this link, and the underlying mechanism that could connect the two conditions "remains unclear," they wrote.

The researchers noted that the study had several limitations, however, in part because the database they used in the research only included information from insurance claims. In other words, the database showed whether people had been diagnosed with dementia or ADHD, but little else. For example, the researchers didn't have information about family history, education levels or diet, and those factors may have played a role.

In addition, the study authors noted that the diagnoses included in the analysis were not made uniformly, and it's possible that some people in the control group had ADHD but were not diagnosed with it, the authors wrote.

To strengthen the current findings, additional studies — particularly studies that use large data sets or national sets — are needed, the researchers wrote.

Originally published on Live Science.