The Pharmacology of Aging

Each day, worldwide, older adults consume millions of doses of medications. This remarkable amount of medication use benefits many older people by preventing and treating disease, preserving functional status, prolonging life, and improving or maintaining good quality of life. However, this level of medication exposure may harm older people via adverse drug reactions and is associated with other problems, such as drug interactions. The responses of older individuals to drugs, both beneficial and harmful, are partially dependent on age-related physiologic changes that influence how the body handles a given drug (pharmacokinetics) and what a drug does to the body (pharmacodynamics). To obtain the desired therapeutic response and prevent drug-related problems, it is also useful to have an understanding of drug use patterns in the geriatric population. Therefore, this chapter first examines the epidemiology of drug use in older adults around the world, followed by age-related alterations in drug pharmacokinetics and pharmacodynamics, and finally drug interactions.

Epidemiology of Drug Use

In general, the number of medications (prescription and nonprescription) used by older adults is greater than the number used by younger persons. In the United States, older adults account for 13% of the population but for 34% of all prescription drugs dispensed. The number and type of medications used by older adults are based in part on their living situation and access to medications.

Living Situation

Community-Living Adults

Of adults aged 57 to 85 years in the United States, 81% have reported taking at least one prescription medication. Although the prevalence of medication users has not changed over time, the prevalence of polypharmacy (the use of multiple medications) has increased in recent years. On average, community-dwelling older adults take from two to nine medications. In the United States, race has been associated with differences in medication use among older adults, with older African Americans and Hispanic Americans demonstrating less use than older whites and Native Americans. Older women also take more medication overall than older men.

Rates of polypharmacy also vary by country. In one international survey of adults 55 years and older, 53% of older adults in the United States reported taking four or more prescription medications. Approximately 40% of older adults in eight other countries—Australia, Canada, Germany, the Netherlands, New Zealand, Norway, Sweden, and the United Kingdom—reported the same medication-taking behavior, and those least likely to report this rate of medication use were from France (29%) and Switzerland (29%).

Also, the use of dietary supplements has been on the rise in the United States, with estimates of use in older adults rising from 14% in 1998 to 49% in 2006. Although dietary supplement use appears to be more common among women than men, rates of nonprescription use overall are similar, with 42% of men and women aged 57 to 85 years in the United States using nonprescription medication. Cardiovascular drugs were found to be the most commonly used medications among all prescription and nonprescription medications in the population studied.

Hospitalized Older Adults

Medication use by hospitalized older adults tends to be slightly higher than that of community-dwelling older adults. However, there is a paucity of information with regard to the types of medications used by older adults in this setting. Reported rates of prescription medication use among hospitalized older adults have ranged from a mean of 5 per patient in Italy and Ireland to 7.5/patient in the United States and Austria. One study, using pharmacy records from the University of Pittsburgh Medical Center, a tertiary academic medical center in southwestern Pennsylvania, identified the top 50 oral drugs prescribed for older hospitalized patients. Warfarin, potassium, and pantoprazole were the most commonly prescribed oral drugs.

Older Adults in Long-Term Care Facilities

The level of medication use by older adults in long-term care facilities (LTCFs) is generally higher than that of older adults living at home in the community. There is a notable disparity worldwide in the percentages of LTCF residents taking large numbers of medications. In the United States and Iceland, 33% of LTCF residents take 7 to 10 medications, whereas only 5% of residents exhibit this degree of use in Denmark, Italy, Japan, and Sweden. In one survey of United States LTCFs, 40% of residents (and 45% of those ≥85 years) received nine or more medications. Gastrointestinal agents, central nervous system agents, and pain relievers were the most commonly used agents among patients receiving polypharmacy in that study.

Although the use of multiple medications may be necessary in some patients, the potential for inappropriate prescribing and drug-related problems are of concern. Overuse of certain centrally active medications—namely, antipsychotics—can be a particular problem in the LTCF setting. In 1987, federal legislation was enacted in the United States that defined clear indications for appropriate prescribing of these agents and mandated close monitoring of them (Omnibus Budget Reconciliation Act [OBRA], 1987). In 2005, the U.S. Food and Drug Administration (FDA) added a black box warning to the labeling of second-generation antipsychotics regarding the increased mortality risk associated with their use in older adults with dementia. This labeling change was then expanded to include all antipsychotics (first and second generation) in 2008. There have been decreases in antipsychotic prescribing in LTCFs since then, but additional efforts are needed to continue to reduce antipsychotic use, particularly among patients at risk of significant harm, such as older adults with dementia.

Access to Medications

Universal public health insurance programs for older adults in Australia, Sweden, Canada, France, Germany, Japan, New Zealand, and the United Kingdom provide some level of drug benefit coverage, with the drug benefits differing in the amount of cost sharing, maximum amount of coverage, and specific pharmaceuticals covered. The U.S. health insurance program for older adults, Medicare, began coverage of outpatient drugs in 2006 via Medicare Part D. Although characterized by substantial copayments and an absence of coverage over a small but fixed drug cost range (the so-called doughnut hole), older adults in the United States are now protected from catastrophic out of pocket costs for outpatient drugs. This, in turn, has improved adherence and reduced the need for older adults to forgo necessities to purchase medications. Notably, in many developing countries, medicines are the largest household health expenditure. Moreover, the supply of medications in developing countries may be inadequate or too expensive for older adults to purchase.

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