Study: Serotonin Dysregulation Is the Common Denominator in Most ADHD Comorbidities

August 13, 2025Serotonin dysfunction occurs in nearly three-quarters of psychiatric and somatic comorbidities linked to ADHD, according to a new systematic review published in Neuroscience & Biobehavioral Reviews.1The study identified 182 comorbidities in ADHD populations and found strong evidence linking 135 (74.2%) of them, including 91 psychiatric and 44 somatic conditions, to imbalances in the brain’s serotonergic system.“This association confirmed the occurrence of comorbidities within a clearly defined ADHD population (rather than the presence of ADHD within a population with a particular condition), and also confirmed that the comorbidity was a co-occurring condition and not an adverse event elicited by ADHD treatment,” the researchers wrote.The research team examined close to 50,000 papers published between January 2010 and March 2024.

Based on the volume of articles and the overall high grade of evidence, they concluded that the top five psychiatric ADHD comorbidities associated with serotonin dysregulation include:The top five somatic ADHD comorbidities associated with a serotonin dysregulation include:The brain’s “feel good” chemical, serotonin is a neurotransmitter that helps regulate mood, impulsivity, and physical functions, such as sleep and digestion.According to a recent study in Nature, serotonin neurons signal to the brain the expected value of near-future rewards.