People with attention-deficit hyperactivity disorder (ADHD) may be more likely to experience gastrointestinal problems, according to a systematic review of 23 studies involving more than 1.9 million people.

The review, led by researchers at the University of Warwick and published in the Journal of Attention Disorders, found that people with ADHD had an almost 50% higher rate of overall gastrointestinal symptoms than those without ADHD. The associations were stronger for some specific conditions, including constipation, irritable bowel syndrome (IBS) and encopresis, or loss of bowel control.

The studies included participants aged between two and 65 years and were conducted in countries including the United States, Sweden, Germany and South Korea. The researchers said the review is the first systematic review focused specifically on gastrointestinal symptoms in people with ADHD.

Co-lead author Sarah Blake, a Medical Student at Warwick Medical School, said: "Almost every gut symptom we looked at: constipation, IBS, even indigestion, appeared to be associated with ADHD. These symptoms can have a real impact on daily life, but they could easily be overlooked if the focus is on ADHD itself."

The researchers said the reasons for the association remain unclear. They identified several possible factors, including impulsivity and irregular eating patterns associated with ADHD, as well as gastrointestinal side effects from some ADHD medications.

They also pointed to evidence of differences in gut bacteria among people with ADHD. One proposed explanation is that changes in gut bacteria could influence communication between the digestive system and brain through the vagus nerve.

The researchers stressed that the findings establish an association rather than showing that ADHD causes gastrointestinal problems. They called for further research to distinguish the effects of ADHD itself from those linked to medication and other factors.

Dr Saran Shantikumar, Associate Clinical Professor at Warwick Medical School, said: "We can't yet say ADHD causes these gut problems, or exactly what is driving the link, but the pattern is striking enough to warrant attention."

The researchers said clinicians treating people with ADHD could consider asking about gastrointestinal symptoms, particularly when they affect quality of life, with some patients potentially requiring specialist dietary or gastrointestinal care.