Furosemide

Mechanism

Furosemide (loop diuretic) increases potassium and magnesium loss. Low potassium and magnesium amplify amiodarone's QT-prolonging effect. Furosemide itself does not change amiodarone levels – the danger is the electrolyte shift.

Symptoms

QT prolongation on ECG. Clinically: dizziness, syncope, palpitations. Severe cases progress to polymorphic ventricular tachycardia (torsades de pointes) with risk of ventricular fibrillation and sudden cardiac death. Risk is higher with hypokalaemia, hypomagnesaemia, bradycardia, and ischaemic heart disease.

Management

The combination is common (atrial fibrillation plus heart failure). Check potassium and magnesium every 1–3 months; if potassium drops below 4.0 mmol/L, add a potassium-sparing diuretic or potassium supplement; if magnesium falls below 0.7 mmol/L, supplement magnesium.

Mechanism

Furosemide increases urinary loss of potassium and magnesium. Digoxin and potassium compete for the same binding site on the Na+/K+-ATPase: as potassium falls, the blocked fraction of the pump grows and the same blood concentration of digoxin produces a larger effect. The digoxin label states it plainly: agents that cause hypokalaemia or intracellular potassium depletion, among them some diuretics, lithium salts and corticosteroids, increase sensitivity to digoxin. Furosemide itself does not change the glycoside concentration; what changes is the electrolyte background.

Symptoms

Nausea, vomiting, loss of appetite and visual disturbance with a yellow or green halo before any cardiac symptom. Then bradycardia, AV block, ventricular ectopy and confusion in older people. Symptoms appear with potassium below 3.5 mmol/L, even when the digoxin level sits inside the therapeutic range.

Management

Potassium and magnesium before the pair starts, at 1-2 weeks and periodically thereafter; sooner with vomiting, diarrhoea or hot weather. If potassium drops below 4 mmol/L in someone on digoxin, it is corrected with potassium or a potassium-sparing agent. Magnesium is replaced below 0.7 mmol/L, because without it hypokalaemia will not correct. The digoxin level is read together with potassium: a normal value against low potassium does not rule out toxicity.

Mechanism

Furosemide (loop diuretic) increases urinary potassium loss. Low potassium amplifies dronedarone's QT effect, which itself carries an FDA boxed warning for this risk.

Symptoms

QT prolongation on ECG. Dizziness, syncope, palpitations. Severe cases: polymorphic ventricular tachycardia (torsades de pointes). Risk is higher with hypokalaemia and hypomagnesaemia.

Management

The combination is common (atrial fibrillation + heart failure). Check potassium and magnesium every 1–3 months; if potassium drops below 4.0 mmol/L, add a potassium-sparing diuretic or potassium supplement.

Mechanism

Gentamicin (an aminoglycoside) is directly oto- and nephrotoxic. Furosemide can also damage the cochlea and reduces renal perfusion via volume depletion. Effects are additive – irreversible sensorineural hearing loss and acute kidney injury have been reported.

Symptoms

Tinnitus, hearing loss, vertigo. Severe cases progress to deafness that does not recover after withdrawal. In parallel: reduced urine output, rising creatinine.

Management

Avoid the combination. If unavoidable, use the minimum effective furosemide dose with daily creatinine and hearing checks (audiometry where possible). Dose gentamicin once daily by weight and renal function, with protocol-driven level monitoring.

Mechanism

Furosemide acts at the loop of Henle and hydrochlorothiazide at the distal tubule. Together they remove more water and sodium and can add to potassium loss (hydrochlorothiazide SmPC).

Symptoms

Weakness, dizziness, thirst, less urine, rising creatinine; in older people, a risk of falls from a sudden drop in blood pressure on standing. Low potassium causes muscle weakness, cramps, palpitations and an irregular heartbeat, which is especially dangerous in patients taking digoxin.

Management

The Spanish hydrochlorothiazide SmPC does not recommend combining it with drugs that cause potassium loss and names furosemide among them. If the combination cannot be avoided, for example in oedema that does not respond to a single diuretic, blood potassium is checked regularly (hydrochlorothiazide SmPC).

Mechanism

Furosemide (loop diuretic) lowers circulating volume and, via subclinical volume depletion, increases lithium reabsorption in the proximal tubules. Serum lithium rises by 25–40%.

Symptoms

Tremor, muscle twitching, nausea, diarrhoea, confusion, ataxia. Severe cases include seizures, coma, and nephrogenic diabetes insipidus with dehydration. Symptoms emerge when serum lithium exceeds 1.2 mmol/L.

Management

Reduce lithium dose by 25–30% when starting or escalating furosemide; check serum lithium at 1 week, then monthly. Maintain normal sodium and fluid balance: deficits worsen lithium toxicity.

Mechanism

Losartan (ARB) reduces aldosterone synthesis and angiotensin-dependent constriction of glomerular afferent arterioles. In a volume-depleted patient on a loop diuretic, the first losartan dose can sharply lower glomerular blood flow and trigger acute kidney injury.

Symptoms

Sharp blood pressure drop on the first dose: dizziness, syncope. Reduced urine output, rising creatinine. Weakness and paraesthesia from hyperkalaemia.

Management

The combination is standard in hypertension and heart failure. Before starting losartan, confirm there is no dehydration (after intensive diuretic therapy). Take the first dose in the evening, lying down. Check potassium and creatinine at 1 week.

Mechanism

Dual mechanism: aspirin competes with furosemide for organic anion tubular secretion in the renal tubules (so furosemide does not reach its target in the loop of Henle lumen), and concurrently suppresses renal prostaglandin synthesis supporting glomerular perfusion.

Symptoms

Reduced urine output, worsening oedema, weight gain. In heart failure: dyspnoea, orthopnoea. Blood pressure and creatinine may rise.

Management

Per the acetylsalicylic acid SmPC, when aspirin and a diuretic are given together, ensure adequate hydration and check renal function (creatinine) at the start of treatment. The risk of acute kidney injury is higher with dehydration or hypovolaemia. The same SmPC links the fall in glomerular filtration to high aspirin doses. At anti-inflammatory doses furosemide diuresis may drop: adjust the furosemide dose by weight, oedema and urine output, or withhold aspirin during decompensation. Furosemide competes with salicylates for renal excretion, so with high-dose aspirin salicylate toxicity can occur at lower doses (FDA Lasix label).

Mechanism

Additive hypotension and fall risk in older patients. No direct pharmacokinetic interaction.

Symptoms

Lower blood pressure, postural dizziness, fatigue. Older patients: fall risk.

Management

In older patients, check standing and sitting blood pressure 1–2 weeks after starting alprazolam. Alternative anxiolytic: hydroxyzine or a short lorazepam course.

Mechanism

Additive orthostatic hypotension. Amitriptyline blocks α1-adrenoceptors and reduces the compensatory vascular response to hypovolaemia (from the diuretic).

Symptoms

Postural dizziness, syncope, fatigue. Older patients: fall and fracture risk.

Management

In older patients, check standing and sitting blood pressure 1–2 weeks after starting amitriptyline. Advise slow rising. For neuropathic pain, alternative: gabapentin or pregabalin (no α1-blockade).

Mechanism

Standard combination in heart failure and hypertension. Additive hypotensive effect; furosemide causes hypokalaemia, amplifying arrhythmia risk with concomitant QT-prolonging drugs.

Symptoms

Lower blood pressure, postural dizziness, fatigue. Older patients: fall risk. With additional antihypertensives: bradycardia (pulse below 60/min).

Management

Standard combination. Check potassium and magnesium every 1–3 months. If potassium drops below 4.0 mmol/L, add a potassium-sparing diuretic (spironolactone) or potassium supplement. Check blood pressure and pulse 1–2 weeks after start.

Mechanism

Additive antihypertensive effect; in hypovolaemic patients it can trigger acute kidney injury. The triad of ACE inhibitor + diuretic + NSAID is especially dangerous (the "triple whammy").

Symptoms

Decreased urine output, rising creatinine, oedema, nausea, fatigue. Higher risk in older patients and in dehydration.

Management

On starting the combination, begin enalapril at the lowest dose (2.5 mg). Check creatinine and potassium at 1–2 weeks, then every 3 months. In older patients with acute illness and dehydration, both drugs are temporarily stopped until volume is restored.

Mechanism

Both cause hypokalaemia: glucocorticoids via mineralocorticoid effect and muscle catabolism; furosemide via direct urinary potassium loss. Additive risk, especially on long-term prednisolone.

Symptoms

Weakness, muscle cramps, fatigue, palpitations, arrhythmia (particularly dangerous with digoxin). Symptoms appear at potassium below 3.5 mmol/L.

Management

For short prednisolone courses (3–7 days), no specific adjustment needed. For long-term therapy, check potassium at 1, 2, and 4 weeks, then monthly. If it falls below 3.5 mmol/L, add potassium supplements (10–20 mmol/day) or a potassium-sparing diuretic. On digoxin, hypokalaemia is dangerous — arrhythmia risk.

Mechanism

Both can cause hyponatraemia: furosemide via natriuresis, sertraline via SIADH-like mechanism. Additive risk, especially in older women with low body weight.

Symptoms

Headache, nausea, confusion, drowsiness; severe cases: seizures. Develops slowly over 1–2 weeks and is often missed in older patients.

Management

Check sodium 2 weeks after sertraline start, then every 6 months. If sodium falls below 130 mmol/L, switch sertraline to mirtazapine or agomelatine (do not cause hyponatraemia). In older patients with moderate hyponatraemia, fluid restriction to 1.5 L/day.

Mechanism

Opposing effects on potassium: furosemide depletes it via the loop of Henle, spironolactone retains it by blocking aldosterone in collecting ducts. Target combination in heart failure, resistant hypertension, and ascites.

Symptoms

With a balanced combination, no specific symptoms. In chronic kidney disease, hyperkalaemia may occur: Weakness, fatigue, paraesthesias, arrhythmia. Symptoms develop at potassium above 5.5 mmol/L; life-threatening above 6.5 mmol/L.

Management

Check potassium and creatinine at 1 and 4 weeks after starting, then every 3 months. With creatinine clearance below 30 mL/min, do not prescribe spironolactone, or cap at 25 mg every other day. In dehydration, temporarily stop both until volume is restored.