The obtained results can be applied in the personalization of duloxetine treatment, i.e., poor and intermediate duloxetine metabolism within the CYP2D6 and concomitant high severity of anxiety symptoms could potentially be considered as a relative contraindication for duloxetine use, while rapid and ultrarapid duloxetine metabolism within CYP2D6 may be potentially associated with a good response in patients with MDD and high severity of comorbid anxiety disorders. The gene discussed is CYP2D6; the disease is Anxiety.