A post argues serotonin is an underdiscussed "third neurotransmitter" in ADHD, alongside dopamine and norepinephrine. That claim traces to a real, very large systematic review published in 2025, and the scale of it is worth walking through.
The real review
Faraone, Ward, Boucher, Elbekai and Brunner published their systematic literature review in Neuroscience and Biobehavioral Reviews in 2025, screening close to 50,000 papers published between January 2010 and March 2024. Their goal was to identify every documented comorbidity occurring in people with ADHD, then check each one against the evidence for a serotonergic link.
What they found
Across 182 identified comorbidities, the review found strong evidence linking 135 of them, 74.2%, including 91 psychiatric and 44 somatic conditions, to imbalances in the brain's serotonergic system. The top psychiatric comorbidities tied to serotonin dysfunction were major depressive disorder, alcoholism, bipolar disorder, autistic disorder and other affective disorders. On the somatic side, the top conditions were irritable bowel syndrome, obesity, migraine, seizures and pain. The authors also confirmed these were genuine co-occurring conditions within a clearly defined ADHD population, not side effects of ADHD medication.
How serotonin fits into the brain
Serotonin is made in a brainstem hub called the dorsal raphe nucleus, which projects out to the prefrontal cortex (impulse control), the amygdala (emotional reactivity), the hippocampus (memory), and the brain's reward circuitry. Along the way, it is synthesized, released, binds to receptors, then gets cleared by a transporter (SERT) or broken down by an enzyme (MAO), each step a place where things can look different in ADHD brains. Serotonin and dopamine also have a push-pull relationship, meaning a shift in one system can ripple into the other, so attention, impulsivity and emotional regulation may reflect an imbalance across multiple interacting systems, not just one.
What this does not mean
This review is about comorbidities, the wide network of conditions that frequently travel alongside ADHD, not a claim that serotonin dysfunction drives the core inattention and hyperactivity symptoms of ADHD itself. Dopamine and norepinephrine remain the most established neurotransmitters for those core symptoms specifically. What this review adds is a large, carefully mapped picture of how often serotonin shows up across the conditions that so often accompany ADHD, depression, alcohol use, bipolar disorder, gut issues, migraine, and more.
Bottom line
The brain rarely runs on a single messenger, and this review is a serious, methodologically large piece of evidence that ADHD may be no exception. If you have noticed your sleep, appetite or mood shifting your focus, that is consistent with a brain where more than one system is involved.
If you want to explore your own neurotype, our free ADHD, depression and bipolar screenings are a starting point for self-understanding, not a diagnosis.
Source: Faraone, S.V., Ward, C.L., Boucher, M., Elbekai, R. and Brunner, E., "Role of serotonin in psychiatric and somatic comorbidities of attention-deficit/hyperactivity disorder: a systematic literature review," Neuroscience and Biobehavioral Reviews (2025). Educational summary, not a diagnosis or medical advice.