Polypharmacy and Drug Interactions

Polypharmacy is the concurrent use of multiple medicines by one person, a situation common among older adults who live with several chronic conditions. As the number of medicines rises, so does the chance that two drugs, or a drug and a disease, will interact in ways that reduce benefit or cause harm. Polypharmacy and the interactions it produces are a central explanation for the high burden of medication-related adverse events in later life.

Definition

Polypharmacy is the simultaneous use of multiple medications by a single patient, most often discussed in older adults; a drug interaction is a modification of one drug's effect by another drug (drug-drug) or by a coexisting condition (drug-disease) that alters efficacy or safety.

Scope

The topic covers what polypharmacy means and why its definition varies, the mechanisms and types of drug-drug and drug-disease interactions, and the association between high medication burden and adverse outcomes. It also notes the tension between single-disease guidelines and the multimorbid older patient. It is a reference and conceptual treatment, not advice on combining or avoiding particular medicines.

Core questions

• How is polypharmacy defined, and why does the threshold vary across studies?
• What are the main types of drug-drug and drug-disease interactions?
• How does increasing medication count relate to adverse drug events?
• Why can guidelines written for single diseases conflict when applied to multimorbid older patients?

Key concepts

• Polypharmacy (numeric and appropriateness-based definitions)
• Drug-drug interactions
• Drug-disease interactions
• Pharmacokinetic vs pharmacodynamic interactions
• Medication burden
• Adverse drug events
• Prescribing cascade
• Multimorbidity
• Single-disease guideline conflict

Mechanisms

Drug interactions arise pharmacokinetically, when one drug changes the absorption, distribution, metabolism, or elimination of another and thereby alters its concentration, or pharmacodynamically, when two drugs act on the same or opposing pathways to amplify or blunt an effect; drug-disease interactions occur when a medicine worsens a coexisting condition. As the number of co-prescribed medicines grows, the number of possible pairwise interactions rises faster than the count of drugs, and the probability of an adverse drug event increases accordingly (Maher, Hanlon, & Hajjar, 2013). Because most clinical guidelines are written for one disease at a time, applying several of them to a multimorbid older patient can generate recommendations that interact or conflict (Dumbreck et al., 2015). A prescribing cascade can also occur, where an adverse effect of one drug is mistaken for a new condition and treated with a further drug.

Clinical relevance

Polypharmacy and drug interactions account for a large share of preventable medication harm in older adults, and recognising them is part of evidence appraisal and medication review in geriatric care. This entry explains the concepts and their evidence base; it is educational background and is not guidance on whether to combine, substitute, or stop any specific medicines.

Epidemiology

Polypharmacy is common in older populations and increases with age and number of chronic conditions, but reported prevalence varies widely because definitions differ; a numeric threshold of five or more regular medicines is frequently but not universally used (Masnoon et al., 2017). Higher medication counts are consistently associated with adverse drug events, falls, and hospitalisation in older adults (Maher, Hanlon, & Hajjar, 2013).

History

Polypharmacy was long treated informally as the use of 'too many' medicines, but systematic work has shown that definitions vary between simple drug counts and appropriateness-based judgements, complicating comparison across studies (Masnoon et al., 2017). As multimorbidity became recognised as the typical state of older patients, attention turned to how single-disease guidelines interact when combined (Dumbreck et al., 2015) and to structured medication review and deprescribing as responses (Scott et al., 2015).

Debates

Should polypharmacy be defined by a number or by appropriateness?

Numeric thresholds such as five or more medicines are easy to measure but treat all extra drugs as equivalent, whereas appropriateness-based definitions distinguish necessary from unnecessary medicines; the field uses both, which complicates comparison across studies.

How should single-disease guidelines be applied to multimorbid patients?

Guidelines are usually written for one condition, so combining several for a multimorbid older patient can produce conflicting or interacting recommendations, raising the question of how to individualise care without abandoning evidence-based standards.

Key figures

• Joseph Hanlon
• Emily Hajjar
• Bruce Guthrie
• Ian Scott

Seminal works

• masnoon-2017
• maher-2013
• dumbreck-2015

Frequently asked questions

Is polypharmacy always harmful?

Not necessarily. Taking several medicines can be appropriate when each is indicated; the concern is unnecessary or interacting medicines, which is why some definitions focus on appropriateness rather than number alone.

What is a prescribing cascade?

It is the situation where an adverse effect of one drug is mistaken for a new medical problem and treated with an additional drug, adding to medication burden and interaction risk rather than addressing the original cause.