Mark Ruscin, PharmD, FCCP, BCPS, Southern Illinois University Edwardsville School of Pharmacy; Sunny A Linnebur, PharmD, BCPS, BCGP, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of Colorado Reviewed / Revised Jul 2021 | Modified Sep 2022 View Patient Education Topic Resources Creatinine Clearance (measured) Glomerular Filtration Rate... Effect of Aging on Metabolism *... Pharmacokinetics is best defined as what the body does to the drug; it includes Absorption Distribution across body compartments Metabolism Excretion With aging, there are changes in all these areas; some changes are more clinically relevant.