Distribution of a given drug may also vary among different persons. For instance, people with large body frames, who have greater amounts of tissue and circulating blood, may require larger amounts of a drug. Obese people may store large amounts of drugs that concentrate in fat, while very thin people may store relatively little. This distribution is also seen in older people, because the proportion of body fat increases with age. All drugs are either metabolized or excreted intact.
Metabolism is the process by which a drug is chemically altered by the body.
The liver is the principal, but not the only, site of drug metabolism. 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.