Drug Interactions in Polypharmacy
Polypharmacy in elderly patients creates exponentially complex interaction networks. Systematic deprescribing and prioritization strategies can reduce adverse drug events by 30-40%.
## Defining Polypharmacy
Polypharmacy is generally defined as concurrent use of 5 or more medications. Approximately 40% of adults over 65 meet this criterion, and 20% take 10 or more drugs. The probability of a clinically significant drug interaction increases exponentially: a patient on 5 drugs has a 50% chance of an interaction, while a patient on 10 drugs faces a near-certain risk.
## The Prescribing Cascade
A prescribing cascade occurs when an adverse drug effect is misidentified as a new condition, prompting an additional prescription:
1. NSAID causes elevated blood pressure
2. Antihypertensive added for "new hypertension"
3. Antihypertensive causes ankle edema
4. Diuretic added for "edema"
5. Diuretic causes hyperuricemia
6. Allopurinol added for "gout"
Recognizing cascades is the first step in safe deprescribing.
## High-Risk Interaction Patterns in Elderly
### Falls Risk Amplification
Multiple CNS-active drugs dramatically increase fall risk:
- Benzodiazepine + opioid + antidepressant + antipsychotic: each additional CNS drug increases fall risk by approximately 30%
- The Beers Criteria identify drugs to avoid or use cautiously in older adults
### Anticholinergic Burden
Many drugs have anticholinergic properties that are individually mild but cumulative. The Anticholinergic Cognitive Burden (ACB) scale quantifies this:
- ACB score 3+: associated with cognitive decline, delirium, falls, and mortality in elderly patients
- Common contributors: oxybutynin, diphenhydramine, paroxetine, amitriptyline, quetiapine
### Renal Elimination Competition
Elderly patients with declining GFR accumulate renally cleared drugs. Adding another renally cleared drug or a nephrotoxin (NSAID, aminoglycoside) to an already-complex regimen creates unpredictable toxicity.
## Systematic Interaction Management
### Medication Reconciliation
At every encounter, verify the complete medication list including OTC drugs, supplements, and herbal products. Studies show the average patient has 3 medication discrepancies at hospital admission.
### Interaction Screening Tools
Clinical decision support systems (CDSS) screen for interactions, but alert fatigue is a major barrier. Only 3-10% of interaction alerts are overridden appropriately. Tiered alert systems that suppress low-severity warnings improve signal-to-noise ratio.
### Deprescribing Frameworks
The STOPP/START criteria and Beers Criteria provide evidence-based guidance:
| Tool | Purpose |
|------|---------|
| STOPP | Screening for potentially inappropriate prescriptions |
| START | Identifying beneficial medications not yet prescribed |
| Beers Criteria | Drugs to avoid in older adults |
| MedSafer | Automated deprescribing opportunity identification |
## Deprescribing Priorities
1. **Drugs causing current adverse effects** — identify and stop the offender
2. **Drugs without current indication** — "why was this started?"
3. **Drugs with unfavorable risk-benefit in current context** — long-term PPI without indication, statin in very elderly with limited life expectancy
4. **Dose optimization** — reduce to minimum effective dose before complete withdrawal
## Key Takeaways
- Interaction probability approaches 100% with 10+ concurrent medications
- Prescribing cascades convert drug side effects into additional prescriptions
- Anticholinergic burden is cumulative; score each drug using the ACB scale
- Medication reconciliation at every encounter catches an average of 3 discrepancies
- Systematic deprescribing using STOPP/START criteria reduces adverse drug events by 30-40%