Drug Interactions & Safety Alerts
Warfarin, digoxin, lithium interactions — QT prolongation, anticholinergic burden, and high-risk combinations
Drug interactions are a leading cause of preventable medication harm in clinical practice. Understanding how drugs affect each other — through pharmacokinetic mechanisms such as CYP enzyme inhibition and induction, or pharmacodynamic mechanisms such as additive QT prolongation — is essential for every prescriber and pharmacist. This article covers the most clinically significant interactions: warfarin, digoxin, and lithium, as well as QT-prolonging drug combinations and anticholinergic burden scoring.
Warfarin Interactions
Warfarin is metabolised by CYP2C9. Drugs that inhibit CYP2C9 raise INR (increasing bleeding risk); drugs that induce CYP2C9 lower INR (increasing clot risk). The most dangerous interaction is with amiodarone — a potent CYP2C9 inhibitor that can double the INR. Other significant inhibitors include metronidazole, clarithromycin, ciprofloxacin, and fluconazole. NSAIDs increase bleeding risk both pharmacodynamically (antiplatelet effect) and through GI mucosal damage. Inducers include rifampicin (the most potent), carbamazepine, phenytoin, phenobarbital, and St John's Wort. Consistent dietary vitamin K intake (leafy green vegetables) is important — erratic intake causes unpredictable INR swings. Check INR more frequently whenever any interacting drug is started or stopped.
Digoxin Interactions
Digoxin has a narrow therapeutic index (0.5–2 nmol/L) and is a P-glycoprotein substrate. Amiodarone is the most important interaction — it reduces renal clearance of digoxin and the digoxin dose must be halved when amiodarone is started. Verapamil, diltiazem, spironolactone, and macrolide antibiotics (clarithromycin, erythromycin) also raise digoxin levels. Even without raising levels, hypokalaemia caused by loop and thiazide diuretics sensitises the myocardium to digoxin toxicity. Classic digoxin toxicity features include nausea, vomiting, yellow-green visual disturbance (xanthopsia), bradycardia, and ventricular arrhythmias. Always check electrolytes — especially potassium and magnesium — before adjusting digoxin therapy.
Lithium Interactions
Lithium is entirely renally eliminated and has a narrow therapeutic index (0.4–1.0 mmol/L). Any drug or condition that reduces renal blood flow or GFR raises lithium levels. NSAIDs — including ibuprofen available over the counter — are a common and often overlooked cause of lithium toxicity, as they reduce renal prostaglandins and GFR. ACE inhibitors, ARBs, and thiazide diuretics similarly raise lithium levels. Patients must be counselled specifically about ibuprofen: to avoid it and to use paracetamol instead. Sick day rules are essential: if the patient experiences vomiting or diarrhoea (causing dehydration), they should stop lithium temporarily and restart when eating and drinking normally. Toxicity above 1.5 mmol/L causes coarse tremor, confusion, and ataxia; above 2.0 mmol/L is a medical emergency requiring ITU review.
QT-Prolonging Drug Combinations
QT prolongation on ECG predisposes to Torsades de Pointes — a potentially fatal ventricular tachycardia. Risk escalates significantly when two or more QT-prolonging drugs are combined. High-risk drug classes include antiarrhythmics (amiodarone, sotalol), antipsychotics (haloperidol, quetiapine, amisulpride), macrolide and fluoroquinolone antibiotics (azithromycin, moxifloxacin), antifungals (fluconazole), certain antidepressants (citalopram at higher doses), and antiemetics (domperidone, IV ondansetron). The MHRA has specifically warned against combining domperidone with azithromycin. Hypokalaemia, hypomagnesaemia, bradycardia, and female sex all worsen QT prolongation risk. Check QTc on ECG before starting high-risk combinations and correct electrolytes first.
Anticholinergic Burden in the Elderly
Anticholinergic burden refers to the cumulative antimuscarinic effect of multiple drugs prescribed simultaneously. In elderly patients — who have reduced cholinergic reserve, lower plasma albumin, and polypharmacy — even low-burden drugs can cause clinically significant harm. High-burden drugs include tricyclic antidepressants (amitriptyline), sedating antihistamines (chlorphenamine, promethazine), oxybutynin for overactive bladder, and hyoscine. Peripheral effects include dry mouth, constipation, urinary retention, blurred vision, and tachycardia. Central effects — more pronounced in the elderly — include confusion, delirium, memory impairment, sedation, and falls. STOPP criteria recommend avoiding anticholinergics in patients with dementia, benign prostatic hyperplasia, and constipation. Where bladder treatment is needed in elderly patients, mirabegron (a beta-3 agonist) has a lower anticholinergic burden and is the preferred alternative.