The Core Physiological Changes
As the body ages, several key physiological changes occur that alter how it processes medications. These changes fall under the umbrella of pharmacology, which is divided into two main areas: pharmacokinetics (what the body does to the drug) and pharmacodynamics (what the drug does to the body). The most impactful changes are related to a reduced capacity for metabolism and elimination.
Pharmacokinetics: How Aging Alters Drug Processing
Pharmacokinetics describes the journey of a drug through the body, including its absorption, distribution, metabolism, and elimination. With age, all four stages can be affected.
Alterations in Drug Absorption
While the overall amount of a drug absorbed often remains stable, the rate of absorption can slow down. Factors contributing to this include a decrease in gastric acid production and slower gastric emptying. For most drugs, these changes do not have a major clinical impact. However, they can alter the timing of a medication's peak effect, which may be a concern for time-sensitive treatments.
Shifts in Drug Distribution
Aging causes shifts in body composition, with an increase in the proportion of body fat and a decrease in total body water and lean muscle mass. This significantly affects drug distribution:
- Fat-soluble drugs: These drugs have a larger volume of distribution due to increased body fat. This can lead to a prolonged half-life, meaning the drug stays in the body longer and may build up to toxic levels with repeated dosing. Examples include benzodiazepines like diazepam.
- Water-soluble drugs: These drugs have a smaller volume of distribution due to decreased total body water. This can result in higher initial drug concentrations in the bloodstream, necessitating a lower starting dose. Examples include digoxin and lithium.
The Impact of Reduced Metabolism
The liver is the primary site for drug metabolism. As a person ages, liver size and hepatic blood flow decrease, and the activity of certain liver enzymes, particularly those in the cytochrome P450 (CYP450) system, may decline. This reduces the liver's ability to break down drugs, particularly those with a high 'first-pass' metabolism. For drugs like certain opioids, this can increase their bioavailability, leading to stronger effects and a higher risk of toxicity.
Impaired Drug Elimination
The kidneys are responsible for eliminating most drugs and their metabolites from the body. Renal function, including glomerular filtration rate, decreases with age. This is often the most clinically significant physiological change affecting medication use. Because lean muscle mass also decreases, standard serum creatinine levels may appear normal despite a substantial reduction in kidney function. Therefore, drug dosages that are cleared by the kidneys, such as certain antibiotics and diuretics, must often be adjusted based on estimated creatinine clearance to prevent accumulation and toxicity.
Pharmacodynamics: How Drug Effects Change with Age
Beyond how the body processes drugs, the way the body responds to them also changes. This is known as pharmacodynamics. In older adults, there is often an increased sensitivity to drugs that affect the central nervous system, such as sedatives, anticholinergics, and pain medications. This can lead to heightened side effects like dizziness, confusion, and falls. Conversely, the body may become less sensitive to certain drugs, like beta-blockers, affecting their therapeutic efficacy.
The Dangers of Polypharmacy and Adverse Events
Due to multiple chronic health conditions, older adults often take numerous medications (polypharmacy), including prescription, over-the-counter, and herbal products. This practice dramatically increases the risk of adverse drug events (ADEs), which can manifest as falls, delirium, and other health complications.
A Comparison of Medication Processing: Younger Adults vs. Older Adults
| Process | Younger Adults (typically) | Older Adults (typically) |
|---|---|---|
| Absorption | Generally faster rates | Slower rates, similar overall amount |
| Distribution | Higher total body water; lower fat | Lower total body water; higher fat |
| Metabolism | High liver mass and blood flow; robust enzyme activity | Reduced liver mass and blood flow; potentially reduced enzyme activity |
| Elimination | Robust renal function; efficient drug clearance | Reduced renal blood flow and glomerular filtration rate; slower drug clearance |
| Side Effects | Often predictable, dose-dependent | Increased risk and severity due to accumulation and altered sensitivity |
Strategies for Safe Medication Use in Seniors
Managing medication safely requires a collaborative approach involving patients, caregivers, and healthcare providers. Awareness of these physiological shifts is the first step.
Best Practices for Seniors and Caregivers
- Maintain a Complete List: Keep an up-to-date list of all medications, including prescriptions, OTCs, vitamins, and supplements. Share this list with every healthcare provider.
- Ask Questions: Always inquire about potential side effects, drug interactions, and any signs of toxicity.
- Monitor Symptoms: Pay close attention to any new or changing symptoms, as they could indicate an adverse drug reaction.
- Review Regularly: Schedule regular medication reviews with a pharmacist or doctor to reassess dosages and identify potentially unnecessary medications.
The Role of Deprescribing
Deprescribing is the process of safely reducing or stopping a medication to reduce the risk of harm or adverse effects, while also maintaining or improving health outcomes. It should always be done in consultation with a healthcare professional. Deprescribing can be particularly important for older adults who are on multiple medications, and it helps to optimize the medication regimen for their current needs.
Conclusion
Understanding what physiological change affecting medication use that occurs with aging is essential for promoting geriatric health and safety. The natural decline in kidney and liver function, coupled with changes in body composition and altered drug sensitivity, creates a complex landscape for medication management. By implementing strategic medication reviews, promoting patient and caregiver awareness, and considering deprescribing where appropriate, healthcare providers can significantly mitigate risks and improve the quality of life for older adults. For more information on navigating medication use as you age, the HealthinAging Foundation provides valuable resources.
