Which of the following is a change in normal older adults that affects drug effectiveness?

A decrease in metabolism is a significant change in normal older adults that affects drug effectiveness. As individuals age, their liver function often declines, leading to reduced metabolic activity. This means that drugs processed through the liver may remain in the system for a longer duration, potentially increasing the risk of drug toxicity and necessitating careful dosage adjustments.

Moreover, this change can alter the therapeutic effects of medications, making it crucial for healthcare providers to monitor older adults more closely when prescribing and administering medications. The slower metabolism can lead to higher plasma concentrations of drugs that are usually metabolized quickly in younger individuals, highlighting the need for individualized approaches to medication management in this population.

Increased absorption of drugs, increased cardiac output, and higher total body water are not typical changes associated with aging in the same manner as decreased metabolism. In fact, aging can often lead to decreased absorption rates, reduced cardiac output, and changes in body composition, such as lower total body water, which can complicate drug dosing further.