The activity of these enzymes is considerably affected by body weight and obesity with CYP3A4 activity being reduced and CYP2E1 activity increased by obesity, as indicated, for instance, by reduced clearance of alfentanyl or triazolam (Cyp3A4 substrates) and an increase in clearance of halothane or chlorzoxazone (Cyp2E1 substrates). This evidence concerns the gene CYP3A4 and obesity due to melanocortin 4 receptor deficiency.