Drug handling: Changes with age

Last updated: Sunday, July 05, 2015

Elderly patients

Pharmacokinetic and pharmacodynamic changes in the elderly may lead to altered drug response. All aspects of pharmacokinetics (absorption, distribution, metabolism and excretion) may be affected, although clinically significant effects due to changes in oral absorption are rare.

Changes in drug distribution are due to changes in body composition with ageing (reduced body water and increased body fat), reduced serum albumin levels, and changes in organ perfusion (reduced liver and kidney blood flow due to decreased cardiac output and increased peripheral vascular resistance).

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Decreased hepatic blood flow leads to a reduction in first-pass metabolism. Together with reduced microsomal oxidation, the capacity of the liver to handle drugs may be significantly impaired in elderly patients. This can lead to increased half-lives and higher steady-state levels of hepatically metabolised drugs.

Renal excretion of drugs may be reduced in the elderly due to a decrease in kidney size and loss of functioning glomeruli. Renal impairment may compound age-related reductions in drug clearance. Note that elderly patients may have a normal plasma creatinine but still have a reduced creatinine clearance – this represents a decreased muscle mass producing less creatinine, compensating declining renal clearance. Reduced renal excretion is particularly significant with drugs of a narrow therapeutic index (e.g. digoxin).

There are also pharmacodynamic changes with age such as an increased sensitivity to the sedating or hypotensive effects of medicines.

Children as patients

Altered pharmacokinetics in paediatric patients may have a significant effect on drug response.

There are probably no significant differences between adults and children in terms of oral absorption of medicines, but the distribution of drugs may be affected by changes in body composition. As a percentage of total body weight, total body water and extracellular fluid volume decrease with age. This is important for water-soluble drugs such as gentamicin, where a larger dose on a milligram per kilogram basis is used in the neonate compared to an older child, to achieve the same plasma concentration.

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In premature babies, plasma protein binding is reduced resulting in higher concentrations of free (active) drug. This also leads to medicines having a higher apparent volume of distribution in premature babies compared to adults.

At birth, the capacity of the neonate to metabolise drugs is significantly lower compared to adults. However, there is a large rise in metabolic capacity in the older infant and young child. For some drugs, such as theophylline, the metabolic clearance in children may be greater than that seen in adults. This necessitates a larger dose on a milligram per kilogram basis to achieve the same plasma concentrations, in children compared to adults. Metabolic pathways that make a minor contribution to adult metabolic capacity may be used more in children to compensate for their less developed systems.

Renal excretion of drugs in young infants is reduced due to anatomical and functional immaturity of the kidneys. However, towards 6-8 months of age, the glomerular and tubular function develops; by 8 months the renal elimination of drugs is approaching that seen in older children and adults.

For more information about the use of medicines in children, follow the link to the tutorial on this subject.