17th May 2025

Physiological changes in the elderly

With increasing age, many physiological changes may alter pharmacokinetics. These include:

• Increased gastric motility
• Reduced intestinal blood flow
• Lower body weight and lean body mass
• Higher proportion of body fat
• Lower hepatic and renal blood flow (and thus glomerular filtration rate)
• Change in immune response

All these factors alter pharmacokinetics:

• Absorption of most drugs remains the same except for reduction in iron, calcium and thiamine absorption
• Lower concentration of fat-soluble drugs
• Higher concentration of water-soluble drugs
• Lower rate of drug metabolism and hepatic clearance
• Less efficient renal drug excretion

Pharmacodynamic changes that also occur simultaneously are as follows:

• Central nervous system becomes more sensitive to sedative medications
• Less effective homeostatic mechanisms and less able to compensate for side effects (especially postural hypotension)
• Reduced clotting factor synthesis
• Increased risk of adverse and allergic reactions

Together with the fact that polypharmacy is more prevalent in the elderly, up to 97% in hospitalised patients, they are also more likely to experience side effects secondary to drug interactions.

Table 1. Common fat-soluble and water-soluble drugs