Researchers at the University of Warwick have analysed data from more than 1.9 million people across 23 studies and found that ADHD is associated with an almost 50% increase in overall gastrointestinal symptoms compared with people without ADHD. The association was even stronger for some conditions, including nearly twice the odds of constipation, a 58% increase in irritable bowel syndrome (IBS), and more than four times the odds of encopresis, or loss of bowel control.

An estimated 5% of children and 3–4% of adults worldwide have ADHD, with reported diagnosis rates rising sharply over the past 40 years. Gut problems are already well documented in autism spectrum disorder, and growing evidence points to a “gut-brain axis” linking digestive health with neurodevelopmental conditions. However, the extent of gastrointestinal symptoms among people with ADHD has remained unclear.

The study, published in the Journal of Attention Disorders, is the first systematic review to focus specifically on gastrointestinal symptoms in people with ADHD. It combined evidence from studies conducted around the world, including the United States, Sweden, Germany and South Korea, and included participants ranging in age from two to 65.

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

The reasons behind the association remain uncertain. Researchers suggest that ADHD-related traits, including impulsivity and irregular eating patterns, could contribute to digestive symptoms. Some ADHD medications can also cause gastrointestinal side effects such as constipation, potentially accounting for part of the observed relationship.

Biological mechanisms may also play a role. Previous research has linked ADHD with differences in gut bacteria, which could potentially influence communication between the digestive system and the brain through pathways including the vagus nerve, a major communication route between the gut and brain.

The researchers say healthcare professionals caring for people with ADHD should be alert to gastrointestinal symptoms and consider asking about them during consultations. People experiencing more complex or persistent symptoms may benefit from additional support from specialists such as dietitians or gastroenterologists, particularly when digestive problems are affecting everyday life and well-being.

However, the researchers emphasise that the findings demonstrate an association and do not prove that ADHD causes gastrointestinal problems. “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,” said Dr Saran Shantikumar, Associate Clinical Professor at Warwick Medical School. “Clinicians caring for people with ADHD may want to consider asking about gastrointestinal symptoms, particularly if they may be affecting quality of life.”

More information: Sarah Blake et al, The Association Between Attention-Deficit Hyperactivity Disorder and Gastrointestinal Symptoms: A Systematic Review and Meta-Analysis, Journal of Attention Disorders. DOI: 10.1177/10870547261469626

Journal information: Journal of Attention Disorders Provided by University of Warwick