CYP3A4 and obesity due to melanocortin 4 receptor deficiency: Although there are a number of reports evaluating the effect of obesity on the activity of some CYP isoenzymes (e.g. CYP3A4, CYP1A2, CYP2C9, CYP2C19, and CYP2D6) most of them, as previously stated, are inconsistent and do not allow conclusions to be drawn5.