Drug Interaction Checker

Drug Interactions: The Complete Clinical Guide

Pharmacokinetic and pharmacodynamic interactions, severity ratings, high-risk combinations, polypharmacy in elderly patients — for doctors, pharmacists and medical students

⚗️ What Are Drug Interactions 🔬 Types of Interactions 🚨 Major Interactions 👴 Elderly & Polypharmacy ✅ Management

What Are Drug Interactions?

A drug interaction occurs when one substance alters the pharmacological activity of another drug — either increasing its effect (potentiation), decreasing it (antagonism), or producing an entirely new, unexpected effect. Drug interactions are responsible for up to 6% of all hospital admissions and are a leading cause of preventable adverse drug events worldwide.

With an ageing population and increasing polypharmacy — patients taking 5 or more medications simultaneously — the clinical significance of drug interactions has never been greater. Identifying and managing interactions is a core prescribing competency for all clinicians.

Types of Drug Interactions

⚗️ Pharmacokinetic (ADME)

One drug affects how another is absorbed, distributed, metabolised or excreted.

Absorption: Antacids reduce ciprofloxacin absorption
Distribution: Warfarin displaced from protein binding by NSAIDs
Metabolism: CYP450 enzyme inhibition/induction
Excretion: NSAIDs reduce renal lithium clearance

💥 Pharmacodynamic

Drugs act on the same receptor or physiological system — effects are additive, synergistic or antagonistic.

Additive: Two CNS depressants → excess sedation
Synergistic: SSRIs + tramadol → serotonin syndrome
Antagonistic: Beta-blockers reduce salbutamol efficacy
Opposing: Anticoagulant + Vit K supplementation

CYP450 Enzyme Interactions — Key Drugs

⬆️ CYP450 Inhibitors (raise drug levels)

Amiodarone, Fluoxetine, Ciprofloxacin
Fluconazole, Erythromycin, Clarithromycin
Omeprazole, Metronidazole, Cimetidine

⬇️ CYP450 Inducers (lower drug levels)

Rifampicin, Carbamazepine, Phenytoin
St John's Wort, Phenobarbitone
Chronic alcohol, Dexamethasone

Major Drug Interactions Every Doctor Must Know

Combination	Risk	Mechanism	Action
Warfarin + NSAIDs	🚨 Major bleeding	Platelet inhibition + protein displacement	Avoid — use paracetamol
SSRIs + Tramadol	🚨 Serotonin syndrome	Additive serotonergic effect	Avoid combination
ACEi/ARB + K-sparing diuretics	🚨 Hyperkalaemia	Additive K+ retention	Monitor K+ closely
Amiodarone + Warfarin	🚨 Bleeding	CYP2C9 inhibition raises INR	Halve warfarin dose, monitor INR
Metformin + Contrast	⚠️ Lactic acidosis	Contrast reduces renal clearance	Stop 48h before contrast
Statins + Macrolides	⚠️ Myopathy	CYP3A4 inhibition raises statin levels	Temporary statin hold
Lithium + NSAIDs	⚠️ Lithium toxicity	NSAIDs reduce renal Li+ excretion	Avoid — monitor levels if unavoidable

Polypharmacy & Elderly Patients

Patients aged over 65 are particularly vulnerable to drug interactions. Reduced renal and hepatic function, altered drug distribution due to body composition changes, and the prevalence of multiple comorbidities all increase interaction risk. Patients on 5 or more drugs have a 50% chance of a clinically significant interaction.

STOPP/START Criteria — Prescribing in the Elderly

STOPP (Screening Tool of Older Persons' Prescriptions) identifies medications that should be stopped in elderly patients. START (Screening Tool to Alert to Right Treatment) identifies medications that should be started. Both tools are recommended by NICE and EMA for medication review in patients over 65.

Managing Drug Interactions in Clinical Practice

✅ Prevention

Check BNF Appendix 1 before prescribing
Use electronic prescribing alerts
Review full medication list at every consultation
Include OTC drugs, herbal remedies, supplements
Ask pharmacist to review complex patients

📊 Severity Ratings

MAJOR — life-threatening, avoid combination
MODERATE — significant risk, monitor closely
MINOR — mild effect, usually manageable
Theoretical — possible but rarely seen clinically

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References

British National Formulary. Appendix 1: Interactions. NICE. Updated March 2026.
Pirmohamed M et al. Adverse drug reactions as cause of admission to hospital. BMJ. 2004;329:15-19.
WHO. Medication Without Harm. Global Patient Safety Challenge. Geneva: WHO; 2017.
O'Mahony D et al. STOPP/START criteria for potentially inappropriate prescribing in older people. Age Ageing. 2015;44(2):213-218.
MHRA. Drug Safety Update. CYP450 interactions. Updated 2024.

⚠️ Medical Disclaimer: This tool is for educational reference only. Always verify interactions with the current BNF or a clinical pharmacist before prescribing.