Prescribing in the elderly

Question 1

Absorption

Answer 1

In old age there is reduced gastric acid secretion. This is accompanied by decreased gastric motility, and reduced surface area available for absorption. Pharmacokinetic studies on the effect of ageing on drug absorption have provided conflicting results. While some studies have not shown significant age-related differences in absorption rates for different drugs, the absorption of vitamin B12, iron and calcium through active transport mechanisms is reduced, whereas the absorption of levodopa is increased.

Question 2

Distribution

Answer 2

There is a relative reduction of body water to body fat (the elderly have more fat and less water) as the body ages which results in an altered distribution of lipid soluble drugs. Older people also have less albumin. These changes result in increased blood levels for water soluble drugs, decreased levels for lipid soluble drugs (increased volume of distribution), and a greater unbound free fraction (increased amount of active drug). As people age, the half-life of a lipid soluble drug increases.

Question 3

Metabolism

Answer 3

Hepatic metabolism of drugs decreases with age as a consequence of reduced hepatic blood flow and reduced enzyme activity.

Question 4

Excretion

Answer 4

The kidneys become less effective with age (by 65, 35% of renal function is lost, and by 80, 50% is lost). Serum creatinine and urea is often used to estimate renal function but can be misleading in the elderly as they have less muscle and so produce less creatinine.

The e-GFR is a tool that can be used to estimate renal function in the elderly.

Drugs that are primarily excreted by the kidneys (e.g. lithium and sulpiride) will tend to accumulate in the elderly and so small doses may be needed.

Question 5

Pharmacodynamics

Answer 5

Receptor sensitivity tends to increase during old age and so this group tends to require smaller doses. It is generally recommended that you start with half of the usual adult dose and go on from there. This increase in sensitivity contributes to an increased incidence of side-effects in the elderly.

Older people tend to take longer to respond to treatment (the therapeutic response is delayed).