Age-related changesAge-related changes in the kidneys, liver, and other organs will influence the way many medications work. Nutritional status, multiple chronic diseases, and functional and cognitive deficits are other age-related factors that may have an impact on drug therapy. In general, because of a loss of muscle mass, elderly persons are physically smaller than younger adults. In addition, the percentage of body fat increases, and body water decreases. Cardiac output (the amount of blood that the heart pumps in one minute) decreases in most elderly persons as well. Kidney function gradually declines, and the effectiveness of the immune system decreases. These changes require a decrease in the dose of some medications to optimize their benefits and avoid toxicity and adverse reactions. Physiologic changes that normally occur with aging may affect the way drugs work within the body.
Drug absorptionWith respect to absorption of drugs, the elderly have a decreased stomach acid and intestinal blood flow. The stomach-emptying time also slows as a person ages. These changes decrease the rate, but not the amount, of drug absorption; this may delay the onset of action and peak effect of medications.
DistributionDrug distribution is the amount of drug that enters various parts of the body tissues, and organs. Some drugs are distributed more easily in the fatty tissue and some in the lean (muscular) tissue. Distribution is affected by normal physiologic changes of aging, which include a higher percentage of fat to lean body mass, a decrease in total body water, and decreased plasma albumin (main protein in the blood). It may be necessary to decrease the dose of some highly fat soluble, water soluble, and highly protein bound medications to compensate for the physiological changes of the aging body. Therefore, fat-soluble (lipophilic) drugs such as valium and phenobarbital will be more widely distributed; this may result in a drug effect which is less intense than expected, but the effect may last longer as a result of slow release of drug from fatty tissue. Since the serum albumin levels decline with aging, this may lead to higher levels of free (not bound to albumin) drugs, therefore a need to lower the normal dose.
MetabolismAging can also affect the ability of the liver to break down drug compounds (metabolism). The elderly have a decrease in liver blood flow, liver size, and enzyme activity. These changes can affect the ability of the liver to break down drugs so that they are easily eliminated. Due to a decrease in liver function, it may be necessary to reduce the dose of some medications that are metabolized by the liver.
EliminationLastly, the aging process affects how drugs are excreted (eliminated) from the body. The elderly have a decrease in kidney function and blood flow to the kidneys. Due to this decrease, it is common to decrease the dose of drugs that are eliminated by the kidneys.
The specific site of action of a drug (drug receptor site) may also change, both in numbers and in sensitivity; this may make the elderly more (or less) sensitive to a drug effect than a younger person; this may produce a more toxic or diminished effect.
The effects of aging as related to drug therapy illustrate the challenges in selecting proper medications for the elderly. Conservative dosing, especially initially, with close clinical monitoring is critical and should be emphasized by all health care practitioners caring for the elderly.