The products of metabolism, metabolites, may be inactive, or they may have similar or different degrees of therapeutic activity or toxicity than the original drug. The liver has enzymes that facilitate chemical reactions such as oxidation, reduction, and hydrolysis of drugs. It has other enzymes that attach substances to the drug, producing reactions called conjugations. Like newborns, the elderly also have reduced enzymatic activity and aren't able to metabolize drugs as well as younger adults and children do. Consequently, newborns and the elderly often need smaller, and children larger, doses per pound of body weight.
Excretion refers to the processes by which the body eliminates a drug.
The kidneys are the major organs of excretion. They are particularly effective in eliminating water-soluble drugs and their metabolites. The kidneys filter drugs from the bloodstream and excrete them into the urine. Many factors can affect the kidneys' ability to excrete drugs. A drug or metabolite must be soluble in water and not bound too tightly to plasma proteins.