As the number of children with attention-deficit/hyperactivity disorder (ADHD) rises, doctors are now searching for an answer to why diagnoses are becoming more common. Last year, a study linked low-level mercury exposure in pregnant women to the disorder. This week, a new study has linked ADHD to childhood allergies and asthma, which are also on the rise.

The study, published in the journal Annals of Allergy, Asthma & Immunology, found that boys who have a history of allergies or asthma are at greater risk for being diagnosed with ADHD. Researchers looked at over 4,400 boys, some of whom had been diagnosed with ADHD. 34% of those with ADHD also had asthma, and 35% had an “allergic disorder.” An intolerance to milk was particularly linked to ADHD.

“ADHD, a chronic mental health disorder, is most commonly found in males, while asthma is also more common in young boys than girls,” said Eelko Hak, lead author of the study and a professor at the University of Groningen. “We found there is an increased risk of ADHD in boys with a history of asthma and an even stronger risk associated with milk intolerance.”

Hak and his colleagues point out that allergies and asthma often run in families, and that the conditions known to be closely linked. They also suggested that medicine used to treat asthma and allergies could be contributing to the rise in ADHD.

“Further research is needed to understand why there appears to be an increased risk of developing ADHD in children with allergy and asthma,” said Dr. Gailen Marshall, editor-in-chief of Annals of Allergy, Asthma & Immunology. “Medications for these conditions far outweigh the risks, and can be life-saving in some conditions. Treatment should not be stopped, unless advised by a board-certified allergist.”