The metabolism and excretion of many drugs decrease with age—requiring that doses of some drugs be decreased. Yet, toxicity may develop slowly because concentrations of chronically used drugs increase for 5 to 6 half-lives, until a steady state is achieved.

This article elaborated that despite an age-related decrease in small-bowel surface area, slowed gastric emptying, and an increase in gastric pH, changes in drug absorption tend to be clinically inconsequential for most drugs. Hence, older adults should use a calcium salt (e.g., calcium citrate) that dissolves more easily in a less acidic environment.

With age, body fat generally increases and total body water decreases. Increased fat increases the volume of distribution for highly lipophilic drugs like diazepam and chlordiazepoxide, and may increase their elimination half-lives.

Important examples of drugs with a high risk of toxic effects include nitrates, propranolol, phenobarbital and nifedipine. Other factors that may influence hepatic metabolism of drugs are – smoking, decreased hepatic blood flow in patients with heart failure and taking drugs that induce or inhibit cytochrome P-450 metabolic enzymes.

Source: MSD Manual. Dec 2018