Drug Absorption

Most medications are taken orally and are absorbed through the gastrointestinal tract. Gastric acid secretion decreases as we age, resulting in an increased gastric pH. When the concentration of acid is lower than normal, drug absorption is reduced. Decreased acidity also affects the breakdown of capsules and tablet coatings in the stomach, resulting in a variable absorption rate, depending on the way a drug is manufactured.

Decreased gastric motility and a slower emptying rate of the stomach are common with aging. These can increase the amount of time that the medication is in contact with the gastric mucosa and can lead to increased absorption. Decreased peristalsis can also affect the speed at which enteric medication reaches the intestine. It may take longer for a drug to reach its site of absorption; therefore, its onset of action may be delayed. Changes in the ability of the cells in the gastrointestinal tract to absorb and transport the drug can further influence its absorption. If medication is not transferred effectively through the cell membrane, the amount of absorption will be decreased.

Drug Distribution

With aging, there is typically a decrease in total body mass, lean body mass, and total body water and an increase in total body fat. These changes can significantly alter the distribution of medications. Because there is less total body water, water-soluble drugs such as gentamicin, histamine-receptor blockers, and lithium tend to remain in higher concentrations in the bloodstream. This results in increased blood concentration levels of these drugs. An older person who is dehydrated is at even greater risk for reaching excessive blood levels of water-soluble drugs.

As muscle mass decreases and the percentage of adipose tissue increases, fat-soluble drugs such as phenobarbital and the benzodiazepines become trapped in the fatty tissue, resulting in abnormally low blood levels. If the dosage is increased based on these blood levels, an excessive amount of medication may be administered. Because fat-soluble drugs continue to be released slowly from the fat into the bloodstream, older persons may exhibit delayed or hangover effects. The half-life of a single dose of diazepam, which is 36 hours in a young adult, may extend to as much as 100 hours in an older individual.

A decrease in hemoglobin and the plasma protein albumin is common with aging. This results in fewer available sites for protein-bound drugs such as warfarin, phenytoin, theophylline, salicylates, and tolbutamide. The danger of adverse or toxic reactions is high even with smaller doses, because an unbound active drug still circulates in the bloodstream. The risk for toxicity is greater in malnourished older adults. Aging persons who consume high-carbohydrate, low-protein diets are more likely to develop toxicity than are aging persons who consume a well-balanced diet. Because not all of the serum drug assays can distinguish between free and bound medications, these tests may not provide reliable measures of toxicity.

Drug Excretion

Aging kidneys are significantly less effective at removing waste products, including the by-products of medications. As the kidneys become less effective in the excretion of drugs, more drug remains in the circulation, leading to elevated drug levels and symptoms of drug toxicity. Circulatory changes that reduce blood flow to the kidneys result in drug accumulation in the bloodstream and increase the risk for toxicity.

Because the changes in kidney function are accompanied by changes in lean body mass, serum creatinine levels often remain constant, masking the decline in function. When the risks for toxicity are assessed, creatinine clearance tests provide a more effective measure of kidney function than does the serum creatinine level.

Medications such as aminoglycosides, digoxin, lithium, procainamide, and cimetidine are likely to reach toxic levels because of poor renal excretion. Non-prescription drugs such as alcohol and nicotine can also affect kidney function and cause changes in drug elimination in older persons.

Polypharmacy

Many factors contribute to the increased usage of medication among older adults, including an increased likelihood of multiple acute or chronic disease conditions, increased availability of a variety of prescription and OTC medications, changes in patient expectations, and changes in the health care delivery system.

Newer, better, and more potent medications are developed every day. Medical conditions of older adults that were once considered untreatable are now treated routinely using medications. Because older adults tend to have more physical complaints or diseases than do younger individuals, medication usage increases exponentially.

Older adults seek medical intervention for many reasons. Some live with their problems and seek medical attention only when they have serious concerns. By the time such a person seeks medical attention, his or her condition may have seriously deteriorated, requiring the prescription of multiple medications. Other older persons make frequent visits to their physicians, seeking reassurance that nothing is seriously wrong. Rather than spending the time needed to reassure older adults, some physicians issue a prescription as a way to terminate the visit. Unfortunately, this poor medical practice occurs too often and can result in older adults taking unnecessary or marginally necessary medications.

Still other older persons expect their physicians to be able to eliminate all of their problems and ailments with medications. Every television show, magazine article, or recommendation from a friend extolling the benefits of a new medication sends some older adults to their doctors’ offices to request or even demand the new medicine. They expect the physician to provide a medication to relieve their ailments, and they often perceive that the physician is not doing anything for them unless some medication is prescribed. Some older adults even go from doctor to doctor until they find one who will give them what they want. Under these pressures, some physicians prescribe medications that they otherwise would not have ordered.

Changes in health care delivery, particularly increased medical specialization, have contributed to medication-related problems. It is increasingly common for an older person to have two or more physicians providing their care. When more than one physician writes prescriptions, the risk for medication reactions and overmedication increases dramatically. If a physician does not know what drugs the patient is already taking, he or she cannot consider those drugs when determining the safety of another prescription. Every physician providing care to an older person must be aware of all medications that person is taking, no matter who prescribed them.