Pharmacologic Management: Analgesic Selection in the Older Patient with Pain

Special Considerations in Treating Pain in the Elderly

Age-related alterations in drug distribution, metabolism and excretion may result in a longer duration of action and greater or lower plasma concentration for many analgesics.^38-40 These changes may lead to increased responsiveness to opioids and other analgesics, with an accompanying increased risk of drug toxicity and adverse reactions.

Perhaps the most clinically significant age-related consideration relates to declines in renal function. In general, renal size, number of functioning glomeruli, glomerular filtration rate, and renal blood flow are all decreased in older patients. These changes can increase the potential toxicity of drugs that are renally-excreted.  While absorption is typically not affected by aging, the volume of drug distribution does change as a result of decreases in lean body weight and total body water, and by alterations in body mass (i.e., increase in adipose tissue). Thus, the volume of distribution for water soluble analgesics, such as morphine, decreases in older patients resulting in an increased effect from a standard loading dose as compared with younger patients. Drug metabolism is changed as well, as a result of decreased liver mass and hepatic blood flow, and the decreased activity of certain drug metabolizing enzymes. The concentration of serum proteins tends to decrease, resulting in reduced amounts of protein-bound (inactive) drug and increased serum concentrations of unbound (active) drug.