4.2 Drug Interactions in Older Adults

Interaction Risk in Aging

Drug interactions are a core pharmacotherapy assessment skill because older adults combine more drugs, more disease states, and less reserve to tolerate toxicity. An interaction that is trivial in a healthy middle-aged adult can precipitate hospitalization in a frail older person. GERO-BC expects you to recognize interaction patterns during assessment—not memorize every pair in a database.

Pharmacodynamic vs. Pharmacokinetic Interactions

Pharmacodynamic interactions dominate many geriatric syndromes: stacked sedation, stacked anticholinergic burden, stacked QT prolongation, and stacked nephrotoxins. Pharmacokinetic interactions matter when narrow-therapeutic-index drugs (warfarin, digoxin, lithium, phenytoin) meet inhibitors, inducers, or renally competing agents.

High-Yield Drug–Drug Patterns

Central nervous system depression

Combining benzodiazepines, nonbenzodiazepine hypnotics (“Z-drugs”), opioids, skeletal muscle relaxants, antipsychotics, and alcohol increases risk of falls, delirium, respiratory depression, and aspiration. Assessment includes sleep aids (including OTC diphenhydramine), liquid formulations containing alcohol, and PRN anxiolytics used nightly.

Anticholinergic burden

Multiple agents with anticholinergic activity (bladder antimuscarinics, first-generation antihistamines, some antidepressants, antipsychotics, antispasmodics) produce confusion, dry mouth, constipation, urinary retention, and tachycardia. Total burden often matters more than any single agent.

Bleeding risk stacks

Ask about aspirin taken for primary prevention without indication, and about fish oil / ginkgo / garlic supplements that may add antiplatelet effects.

Cardiovascular and electrolyte interactions

• ACE inhibitor or ARB + potassium-sparing diuretic / potassium supplement / trimethoprim → hyperkalemia
• ACEI/ARB + NSAID + diuretic (“triple whammy”) → acute kidney injury
• Non-DHP calcium-channel blocker + beta-blocker → bradycardia, heart block risk
• Digoxin + amiodarone / verapamil / clarithromycin → digoxin toxicity (PK + PD)

QT prolongation

Additive QT risk from macrolides, fluoroquinolones, certain antipsychotics, antiemetics (e.g., ondansetron in susceptible patients), and methadone—especially with electrolyte depletion (diuretics). Assessment notes recent antibiotics plus psychiatric meds and checks for syncope or palpitations.

Serotonergic toxicity

SSRIs/SNRIs combined with MAOIs (rare but critical), linezolid, tramadol, triptans, or St. John’s wort raise concern for serotonin syndrome—agitation, clonus, hyperreflexia, diaphoresis, fever. Older adults may present atypically; medication list review is essential when mental-status changes appear after a new serotonergic agent.

Drug–Food and Drug–Herbal Interactions

Assessment must include diet and supplements:

• Warfarin–vitamin K consistency (not absolute avoidance): leafy greens alter INR if intake fluctuates
• Grapefruit juice inhibits CYP3A4 → ↑ levels of some calcium-channel blockers, statins, and other substrates
• Calcium / iron / magnesium / antacids bind tetracyclines, quinolones, levothyroxine, and bisphosphonates → reduced absorption if co-administered
• St. John’s wort induces CYP3A4/P-gp → loss of efficacy of many drugs (including some anticoagulants and immunosuppressants)
• Licorice root, high-dose vitamin E, and certain herbals may affect blood pressure or clotting

Assessment Workflow for Interactions

1. Build the complete list (Section 4.1), including OTCs and herbals
2. Flag narrow-therapeutic-index and high-risk drug classes
3. Look for additive toxicity themes (sedation, anticholinergic, bleed, QT, nephrotoxin, serotonergic)
4. Identify CYP / transporter red flags when a new antibiotic, antifungal, or amiodarone is started
5. Correlate timing: new confusion, bleed, bradycardia, or AKI after a regimen change
6. Communicate specific interaction concerns to the prescriber with clinical findings attached

Timing Clues

Many interaction harms appear days to weeks after a new drug is added (antibiotic + warfarin; NSAID + ACEI/diuretic). Ask: “What changed in the past two weeks?” Include stopped drugs (withdrawal) and started herbals from well-meaning family members.

Over-the-Counter Sleep and Cold Products

First-generation antihistamines in nighttime pain/sleep combinations and cold remedies add anticholinergic and sedative load that patients rarely volunteer unless asked by product name. Pair this question with fall and delirium screening whenever a new “PM” analgesic appears on the brown-bag list.

Documentation and Hand-off

When assessment uncovers an interaction risk, document:

1. The specific agents and suspected mechanism family (PD stack vs. PK exposure change)
2. The clinical finding that raises concern (INR rise, creatinine bump, falls, delirium)
3. Timing relative to regimen changes
4. Patient/caregiver understanding and any self-started OTCs or herbals

Clear hand-off prevents the next clinician from attributing the syndrome solely to “aging” or a new medical diagnosis. In transitions of care, explicitly list high-risk pairs still active at discharge.

Alcohol and Nicotine Products

Ask about alcohol quantity and pattern—even “social” intake interacts with sedatives, anticoagulants, and hepatically metabolized drugs. Nicotine replacement and smoking status can alter CYP1A2 substrates (e.g., some antipsychotics, theophylline historically). These are part of interaction assessment, not optional social history.

Exam tip: Prefer interaction mechanism families over rote lists. If the stem shows polypharmacy plus a new antibiotic, bleeding, or AKI, search for stacked PD effects or classic PK pairs before assuming primary disease alone.