Due to the lack of information on the possible effect of low body weight on most CYP enzymes, the aim of this study was to explore whether changes in body weight in patients with anorexia nervosa affect the metabolic activity of the CYP enzymes CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4. This evidence concerns the gene CYP2D6 and anorexia nervosa.