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In page Attention deficit hyperactivity disorder:

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Reviews of ADHD biomarkers have noted that platelet monoamine oxidase expression, urinary norepinephrine, urinary MHPG, and urinary phenethylamine levels consistently differ between ADHD individuals and non-ADHD controls in aggregate.[citation needed] These parameters could serve as prognostic biomarkers for ADHD, but more research is needed to establish their prognostic utility.[1] Urinary and blood plasma phenethylamine concentrations are lower in ADHD individuals relative to controls.[2][3] The two most commonly prescribed drugs for ADHD, amphetamine and methylphenidate, increase phenethylamine biosynthesis in treatment-responsive individuals with ADHD.[4] Lower urinary phenethylamine concentrations are associated with symptoms of inattentiveness in ADHD individuals.[5]