Factors related to age, such as renal clearance, increased metabolic rate, and higher food intake relative to body weight, also affect the concentration and have an effect on drug metabolism.
Enzyme activity naturally declines gradually with advancing age due to loss of efficiency in the systems that control metabolic processes. The mass of the liver, as well as its blood flow, decreases with age. There is a gradual decline in renal function and, therefore, clearance. With decreased first-pass metabolism, blood levels of a drug can be quite high.
Many elderly patients take multiple prescriptions. Evaluating possible drug-drug interactions in the geriatric patient can be quite challenging for the physician.
CYP2D6 and most other CYPs, (with the exception of CYP1A2,) are not significantly affected by age. However, elderly people have reduced liver blood flow and therefore reduced phase I metabolism. This results in higher blood levels of the parent drug for elderly patients, as compared to their younger counterparts.
Once UDP-glucuronyl transferase concentrations reach mature levels, glucuronidation is no longer affected by age.
