Pharmacokinetics in Geriatric Patients: Effect of Age on Drug Absorption

Aging increases gastric pH, delays gastric emptying, and reduces gastrointestinal motility. However, these changes do not substantially alter oral drug absorption, particularly for drugs absorbed via passive diffusion. This means older adults typically maintain normal absorption of orally administered medications despite age-related gastrointestinal changes.

Transdermal delivery offers unaffected drug absorption in elderly patients and reduces adverse effects, making it ideal for managing chronic pain and neurological conditions. Age does not significantly alter drug absorption through transdermal systems, though individual drugs like fentanyl may require dosage adjustments. The method provides prolonged therapeutic effects and controlled drug release from the patch.

Drug molecule size is a key determinant of absorption pathway in subcutaneous delivery. Smaller molecules pass through vascular capillaries, while larger molecules travel via lymphatic channels. Age-related changes in vascular capillaries and lymphatic channels, along with altered skin blood supply and lymphatic drainage, influence this route's effectiveness in elderly patients.

Older adults experience enlarged alveoli, decreased lung elasticity, reduced capillary volume, and increased residual volume, all affecting inhaled drug absorption. These anatomical and physiological changes require lower doses of inhaled anesthetic drugs like isoflurane and sevoflurane in elderly patients compared to younger adults to achieve therapeutic effects.

Age-associated modifications in skin hydration and lipid content enhance the stratum corneum's barrier function, particularly affecting hydrophilic compounds. Lipophilic drugs maintain their absorption efficiency despite these changes. These skin alterations influence how different drug types penetrate and absorb through transdermal systems in older individuals.

Age-related alterations in the cornea and Bruch's membrane thickness affect drug permeability from the sub-conjunctival space into the vitreous humor. These changes are particularly significant for older patients with ocular diseases such as macular degeneration and glaucoma, potentially requiring adjusted dosing strategies for optimal therapeutic outcomes.

Research indicates that intramuscular drug absorption is quite similar to the subcutaneous route and remains consistent across different ages. Studies on drugs like diazepam and midazolam show comparable absorption patterns in elderly and younger patients, suggesting age does not substantially alter this particular drug delivery route.