Nine Reasons Why Older Adults Are More Likely Than Younger Adults to Have Adverse Drug Reactions

There are significant differences in the ability of younger and older patients to process drugs, a fact that many doctors and patients do not realize. It is not surprising to hear that drug-induced diseases are more common in people aged 60 and older, since many of the studies and much of the information concerning the epidemic of drug-induced disease focus on this age group. For many drugs, older adults should be started on lower doses and kept on the minimal number of drugs necessary. But you might not know why older adults are more likely than younger adults to experience adverse drug reactions. This article explains why.

1. Smaller Bodies and Different Body Composition

Older adults generally weigh less than younger adults, and have a smaller amount of water and a larger proportion of fat. Body weight typically increases from age 40 to 60, mainly due to increased fat, then decreases from age 60 to 70, with even sharper declines from 70 on. Therefore, the amount of a drug per pound of body weight or body water will often be much higher in an older adult than it would be if the same amount of the drug were given to a younger person. In addition, drugs that concentrate in fat tissue may stay in the body longer because older adults have more fat tissue in which the drugs can accumulate.

2. Decreased Ability of the Liver to Process Drugs

Because the liver does not work as well in older adults, they are less able to process certain drugs so the drugs can therefore be excreted from the body less efficiently. This has important consequences for a large proportion of the drugs used to treat heart conditions and high blood pressure, as well as many other drugs processed by the liver. The ability of the body to rid itself of drugs such as diazepam (VALIUM), chlordiazepoxide (LIBRIUM) and many others is affected by this decrease in liver function.

3. Decreased Ability of the Kidneys to Remove Drugs From the Body

The ability of the kidneys to excrete drugs through the urine begins to decrease steadily starting around age 35 or 40. By age 65, the filtering ability of the kidneys has already decreased by 30 percent. This has an effect on the safety of a large number of drugs. A given dose of the drug may not be excreted as efficiently, resulting in higher blood levels—even though the same dose might be safe in younger people—and, possibly, increased side effects.

4. Increased Sensitivity to Many Drugs

The aforementioned problems can cause many drugs to accumulate in older people’s bodies at dangerously high levels and for longer times than in younger people. These age-related problems are further worsened by the fact that even at “normal” blood levels of many drugs, older adults have an increased sensitivity to their effects, often resulting in harm. This is seen most clearly with drugs that act on the central nervous system, such as many sleeping pills, alcohol, tranquilizers, strong painkillers such as morphine or pentazocine (TALWIN), and most drugs that have anticholinergic effects, including dry mouth, constipation, difficulty urinating, confusion, worsening of glaucoma, blurred vision and short-term memory problems. Anticholinergic drugs include antidepressants, antipsychotic drugs, antihistamines, drugs used to calm the intestinal tract (for treating ulcers or some kinds of colitis) such as the combination of atropine, hyoscyamine, scopolamine and phenobarbital (DONNATAL), atropine, the combination of chlordiazepoxide and clidinium (LIBRAX), antiparkinsonian drugs, and other drugs such as disopyramide (NORPACE).

Another example of the marked increase in the sensitivity of older adults to drugs has to do with stimulant drugs that are in the same family as amphetamines, or “speed.” Despite the dangers of these drugs for anyone, especially older adults, they are widely promoted and prescribed, including triprolidine and pseudoephedrine (ACTIFED), which contain the amphetamine-like drug pseudoephedrine. Any of these drugs discussed on www.worstpills.org (most of which are listed as Do Not Use drugs) contains a warning about the potential for these drugs to cause or worsen high blood pressure. The warning says these drugs are especially dangerous for people with high blood pressure, heart disease, diabetes or thyroid disease, and that people over 60 are more likely than younger people to experience effects on the heart and blood pressure, restlessness, nervousness and confusion. (To see the full warning, visit www.worstpills.org.)

5. Decreased Ability to Maintain Blood Pressure

Older adults also are less able to maintain their blood pressures when they use certain drugs, including high blood pressure drugs. The most widespread example of older adults’ decreased ability to compensate is seen when they get out of bed and/or suddenly rise from a seated position. As one rises, one’s blood pressure normally falls, decreasing the blood flow to your head and resulting in less blood flow to the brain. Younger people’s bodies can compensate for this: Receptors in the neck, sensing that the blood pressure is falling as the person stands up, tighten up the blood vessels in other parts of the body, thus keeping the overall blood pressure high enough. In older adults, these receptors do not work as well. Often, upon standing, older adults feel giddy, lightheaded, and dizzy. They may even faint because the blood pressure in the head falls too rapidly.

High blood pressure drugs— because they lower blood pressure —are a particular problem in this regard. Other categories of drugs that cause an exaggerated blood pressure drop include sleeping pills, tranquilizers, antidepressants, antipsychotic drugs, antihistamines, drugs for heart pain (angina) and antiarrhythmics. (To see a full list of drugs that can cause dizziness upon standing, visit www.worstpills.org.)

This problem of so-called postural hypotension — the sudden fall in blood pressure upon standing — can be catastrophic. The falls that often result can end in hip fractures, a leading cause of death in older adults, or other serious injuries.

6. Decreased Temperature Compensation

Younger adults are more easily able than older people to withstand very high or very low temperatures. They sweat and dilate (widen) blood vessels to get rid of excess heat when it is hot, and constrict (narrow) blood vessels to conserve heat when it is cold. Older adults’ are less able to do this. As in the case of blood pressure compensation, this “normal” temperature-regulating problem of older adults can be significantly worsened by any of a large number of prescription and over-the-counter drugs, resulting in fatal or life-threatening changes in body temperature. In an article soon to appear we will discuss and list the drugs most likely to impair temperature regulation by inhibiting sweating. Many older adults’ deaths during heat waves or prolonged cold spells can be attributed to drugs that interfere with temperature regulation. Most of these people did not know they were at increased risk. All drugs on www.worstpills.org with a warning about anticholinergic effects can have this harmful effect.

7. More Likely to Have Diseases That Affect the Response to Drugs

Older adults are much more likely than younger adults to have at least one disease — such as liver or kidney damage (not just the decreased function of older age), poor circulation and other chronic conditions — that alters their response to drugs. Little is known about the influence of multiple diseases on drug effects in the elderly.

One well-understood example is the effect of heart failure on the way people can handle drugs. When the heart is not able to pump as much blood as it used to, the change that occurs with heart failure, there is also a decrease in the flow of blood to the kidneys, decreasing the kidneys’ ability to rid drugs from the blood and excrete them in the urine.

8. More Likely to Take Multiple Drugs

Because older adults use significantly more prescription drugs than younger people, they have greatly increased risk of having a drug reaction caused by the dangerous interaction between two drugs. Often, older adults take one or more over-the-counter drugs in addition to their prescription drugs. This further increases the likelihood of adverse drug interactions. The most common kind of adverse drug interaction is the ability of one drug to cause a second drug to accumulate to dangerous levels in the body. At the end of the discussion of each drug on www.worstpills.org, except for the Do Not Use drugs, there is a list of other drugs that can cause serious adverse interactions.

9. Inadequate Testing of Drugs in Older Adults Before Approval

Although older adults use a disproportionate share of prescription drugs, few of these drugs are adequately tested in older adults before being approved by the Food and Drug Administration (FDA).

The FDA is slowly remedying this serious problem by requiring that the people on whom a drug is tested be representative of those who will use the drug if it is approved. Nonetheless, most drugs on the market today were not adequately tested in this age group.

In summary, older patients are at increased risk for adverse drug reactions. Increasing awareness of this problem should result in the prescription of far fewer drugs to older adults, and those that are prescribed will be given at lower doses in most instances.