Developmental Changes in How Medications Work
When Lucy takes her medications every day, what happens? Understanding how medications work involves knowing the developmental changes in absorption, distribution, metabolism, and excretion of medications (Hacker, Messer, & Bachmann, 2009).
Absorption is the time needed for one of Lucy’s medications to enter the bloodstream. For drugs taken orally, a key factor is the time it takes for the medication to go from the stomach to the small intestine, where maximum absorption occurs. This transfer may take longer than expected in older adults, resulting in too little or too much absorption, depending on the drug. For example, if a drug takes longer to transfer from the stomach to the small intestine in older adults, too little of the drug may be left to be effective. However, once in the small intestine, absorption does not appear to differ among older, middle-aged, or younger adults (Hacker et al., 2009).
Once in the bloodstream, the medication is distributed throughout the body. How well distribution occurs depends on the adequacy of the cardiovascular system. Maximal effectiveness of a drug depends on the balance between the portions of the drug that bind with plasma protein and the portions that remain free. As we grow older, more portions of the drug remain free; this means that toxic levels of a drug can build up more easily in older adults. Similarly, drugs that are soluble in water or fat tissue can also build up more easily in older adults because of age-related decreases in total body water or
possible increases in fat tissue. The effective dosage of a drug depends critically on the amount of free drug in the body; thus, whether the person is young or old, thin or obese, is very important to keep in mind (Hacker et al., 2009).
Getting rid of medications in the bloodstream is partly the job of the liver, a process called drug metabolism. There is much evidence that this process is slower in older adults, meaning that drugs stay in the body longer as people grow older (Cusack, 1995). Slower drug metabolism can also create the potential for toxicity if the medication schedule does not take this into account. Sometimes drugs are decomposed into other compounds to help eliminate them. Drug excretion occurs mainly through the kidneys in urine, although some elimination occurs through feces, sweat, and saliva. Changes in kidney function with age, related to lower total body water content, are common. This means that drugs often are not excreted as quickly by older adults, again setting the stage for possible toxic effects (Hacker et al., 2009).
What do these changes mean? Most important, the dosage of a drug needed to get the desired effect may be different for older adults than for middle – aged or younger adults. In many cases, physicians recommend using one third to one half the usual adult dosage when the difference between the effective dosages and toxic dosages is small or there is a high rate of side effects (Hacker et al., 2009). In addition, because of age-related physiological changes, several drugs are not recommended for use by older adults. In general, a dosage strategy of “start low and go slow” is best.