SLC6A3 and attention deficit-hyperactivity disorder: These results are consistent with the use of modafinil and methylphenidate as cognitive enhancers and to treat symptoms of ADHD: both drugs engage the dopaminergic system by blocking dopamine transporter as one of their main mechanisms of action [38, 46, 64, 65, 74, 83, 87, 95, 135, 146, 152–154, 157].