Of those in the study, boys newly diagnosed with ADHD were 40% more likely to have asthma, 50% more likely to have needed a prescription for allergy medicine and 50% more likely to have had a bacterial skin infection than other boys.
"Our study provides additional evidence to support the hypothesis that atopic disorders, such as asthma and food allergies increase the risk of developing ADHD," the authors wrote, adding that further research is necessary to determine just how these conditions might be connected.
Their results were published in the August issue of the Annals of Allergy, Asthma & Immunology.
ADHD, a chronic mental health condition involving difficulty paying attention, hyperactivity and impulsivity, affects as many as 9% of American children, according to background information in the study conducted by Eelko Hak, of the University of Groningen, and colleagues in the Netherlands and Boston.
ADHD increasing along with allergies
The increase in the prevalence of ADHD has been paralleled by an increase in allergic (also called atopic) diseases, such as asthma and allergies, the researchers reported. They also noted that environmental risk factors, such as foods that cause an allergic reaction, may trigger symptoms of both ADHD and allergic asthma.
To get a better idea of whether or not there actually is an association between these conditions, the researchers used data from a large UK study. Within that database, the researchers found nearly 900 boys who were first diagnosed with ADHD and prescribed medication for the condition between 1996 and 2006.
All of the boys were between 4 and 14 years old when first diagnosed.
The researchers compared the children with ADHD to about 3 500 children without the condition.
After adjusting the data to account for age, and for low birth weight or premature birth, they found significant relationships between the diagnosis of ADHD and a history of asthma, impetigo or a prescription for antihistamines (allergy medicines).
They also found weaker associations between ADHD and cow's milk intolerance, and prescriptions for oral or topical corticosteroids, antibacterial or antifungal drugs.
The authors theorise that the links they found may be food-allergy related. However, this study didn't attempt to prove cause and effect, so the exact reason behind the association remains unknown.
Dr Andrew Adesman, chief of developmental and behavioural paediatrics at Steven and Alexandra Cohen Children's Medical Center of New York, said the connection between ADHD and allergic diseases has been seen in other studies.
"The association seems to be real. The chicken-and-the-egg question remains unanswered. The challenge is in teasing out why they're linked," he said.
For her part, Dr Jennifer Appleyard, chief of allergy and immunology at St. John Hospital and Medical Center in Detroit, said, "This is an interesting, but very early study. They're definitely not showing cause and effect."
Appleyard pointed out that impetigo and milk intolerance aren't typically considered allergic diseases. Impetigo is a bacterial infection of the skin. And, a milk intolerance isn't the same as an allergy to milk.
"They looked at food allergies, too, and they didn't find an association. They also didn't find an association with atopic dermatitis [eczema], and impetigo is not necessarily correlated with an allergic reaction," she said.
The bottom line, she said, is that parents don't need to have any additional fears from this study. She added that parents of children with asthma or allergies shouldn't start worrying that their children will develop ADHD – and parents definitely shouldn't make any changes to medications because of this study.
"All of these conditions seem to have increased. Let's pursue this link further, but there's no need for any changes right now," Appleyard said.