Hydrochlorothiazide

Mechanism

Hydrochlorothiazide causes hypokalaemia and hypomagnesaemia; combined with dofetilide leads to torsades de pointes. FDA Tikosyn Section 4.3 strict contraindication.

Symptoms

Torsades de pointes, syncope, cardiac arrest.

Management

Combination contraindicated. Use loop diuretic with potassium monitoring.

Mechanism

Hydrochlorothiazide (thiazide diuretic) increases potassium and magnesium loss. Low levels amplify amiodarone's QT-prolonging effect.

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 (hypertension plus atrial fibrillation). Check potassium and magnesium every 3 months; if potassium drops below 4.0 mmol/L, add a potassium-sparing diuretic or potassium supplement.

Mechanism

Both cause renal potassium loss. Dual hypokalaemia with arrhythmia risk.

Symptoms

K+ below 3.0 mmol/L, weakness, arrhythmia.

Management

Daily potassium check, potassium and magnesium replacement.

Mechanism

Thiazides reduce urinary calcium excretion: that property is exploited in calcium stone disease, but it means calcium is retained. If vitamin D simultaneously increases what comes in through the gut, the balance shifts towards hypercalcaemia. Risk rises with age, with summer dehydration and with reduced renal function.

Symptoms

Weakness, nausea, constipation, polyuria and thirst, confusion in the older patient. Moderate hypercalcaemia is easily mistaken for the diuretic's own effect or for dehydration.

Management

This is not a forbidden combination: many patients with hypertension need vitamin D. What changes is the follow-up. Measure serum calcium before starting vitamin D and at 4–8 weeks, then periodically. Avoid high vitamin D doses without a documented deficiency and watch hydration in summer. If calcium rises, adjust the supplement first, not the antihypertensive.

Mechanism

The hydrochlorothiazide label gives digitalis glycosides their own entry: thiazide-induced hypokalaemia or hypomagnesaemia can favour digitalis-induced arrhythmia. The mechanism matches the one with loop diuretics – competition between digoxin and potassium at the Na+/K+-ATPase – and the digoxin label lists diuretics among agents that raise sensitivity to the glycoside. The thiazide label also asks for hypokalaemia and hypomagnesaemia to be corrected before treatment starts.

Symptoms

Nausea, vomiting, poor appetite, yellow-haloed vision, confusion. On ECG, ventricular ectopy, bradycardia and AV block. Muscle weakness and cramps point to low potassium in their own right, before any digitalis picture appears.

Management

Potassium and magnesium before the thiazide starts and during follow-up. The label asks for hypokalaemia and hypomagnesaemia to be corrected first and for hydrochlorothiazide to be stopped if hypokalaemia comes with muscle weakness, paresis or ECG changes. In a patient on digoxin the potassium target stays above 4 mmol/L; potassium supplementation, a potassium-sparing agent or a change of diuretic all serve.

Mechanism

Hydrochlorothiazide (thiazide diuretic) increases urinary potassium loss. Low potassium amplifies dronedarone's QT effect.

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 (hypertension + atrial fibrillation). Check potassium and magnesium every 3 months; if potassium drops below 4.0 mmol/L, add a potassium-sparing diuretic or potassium supplement.

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

Hydrochlorothiazide increases sodium and lithium reabsorption in the distal tubule. Lithium plasma concentration rises 25–40% (up to 100% in some patients). Lithium toxicity develops within 1–2 weeks.

Symptoms

Tremor, nausea, diarrhea, confusion; in severe cases seizures and coma.

Management

For hypertension in a lithium patient, choose another class: calcium channel blocker (amlodipine) or a potassium-sparing diuretic with caution. If thiazide is required, halve lithium and check level on days 5–7 and at 2 weeks.

Mechanism

A thiazide diuretic plus an ARB synergistically lower blood pressure: the thiazide reduces circulating volume, losartan vasodilates by suppressing the RAAS. This is a target combination in hypertension and available as a fixed-dose product (losartan + hydrochlorothiazide).

Symptoms

Excessive blood pressure drop during titration: postural dizziness, syncope. Hypokalaemia and hyponatraemia from the thiazide, plus a mild rise in creatinine, can occur in the first weeks.

Management

The combination is a treatment standard. Doses are individualised, usually starting with losartan 50 mg + hydrochlorothiazide 12.5 mg/day. Check potassium, sodium, and creatinine 2–4 weeks after start, then annually.

Mechanism

Thiazide diuretics raise allopurinol and oxypurinol metabolite levels, increasing hypersensitivity risk, including allopurinol hypersensitivity syndrome (DRESS).

Symptoms

Rash, fever, facial oedema, lymphadenopathy, abnormal liver function. Reactions develop 2–8 weeks after starting allopurinol, more often in older patients and chronic kidney disease.

Management

On the combination, dose allopurinol by creatinine clearance: if below 60 mL/min, start at 100 mg/day with titration of 100 mg every 4 weeks until target uric acid is reached. In Asian patients (especially Han Chinese and Thai), HLA-B*58:01 testing is recommended before starting allopurinol. Alternatives in hyperuricaemia: febuxostat (if no heart failure), or replace the thiazide with losartan (urocosuric effect).

Mechanism

Additive hypotension and fall risk. 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 start. Alternative anxiolytic: hydroxyzine or a short lorazepam course.

Mechanism

Additive orthostatic hypotension. Older patients: fall and delirium risk with thiazide-induced hypovolaemia.

Symptoms

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

Management

In older patients, check standing and sitting blood pressure and assess fluid and electrolyte balance. For neuropathic pain, alternative: gabapentin or pregabalin.

Mechanism

Standard combination in hypertension – additive antihypertensive effect. Thiazide diuretic causes hypokalaemia.

Symptoms

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

Management

Standard combination (fixed-dose product available). Check potassium 2–4 weeks after start and then annually. If potassium drops below 4.0 mmol/L, add a potassium-sparing component.

Mechanism

Additive antihypertensive effect. In the first weeks hydrochlorothiazide increases renin-angiotensin system activity, raising sensitivity to the ACE inhibitor. A standard fixed combination for arterial hypertension.

Symptoms

Symptomatic hypotension on starting: dizziness, weakness, especially on standing. In older patients: fall risk. Potassium monitoring matters — thiazide depletes, ACE inhibitor retains.

Management

On starting the combination, begin enalapril at the lowest dose (2.5 mg) at bedtime. Check creatinine and potassium at 1–2 weeks, then every 6 months. In older patients, take the first dose seated.

Mechanism

Thiazide diuretics reduce glycaemic control via hypokalaemia-driven suppression of insulin secretion. HbA1c may rise by 0.3–0.5%, especially in the first 6 months.

Symptoms

Return of thirst and polyuria, rising fasting glucose, signs of worsening control. Symptoms are typically mild.

Management

Check fasting glucose and HbA1c 3 months after starting hydrochlorothiazide. If HbA1c rises, increase metformin or add another glucose-lowering agent. Potassium every 6 months. In overt diabetes decompensation, alternatives: indapamide (less metabolic effect) or an SGLT-2 inhibitor.

Mechanism

Thiazides are one of the most common drug causes of hyponatraemia in older patients via SIADH-like mechanism and natriuresis. SSRIs (sertraline) add a SIADH-like effect. Additive risk.

Symptoms

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

Management

On the combination, check sodium at 2 and 4 weeks after starting sertraline or the thiazide, then every 6 months. If sodium falls below 130 mmol/L, consider switching sertraline to mirtazapine or agomelatine, or replacing the thiazide with indapamide (lower hyponatraemia risk).

Mechanism

Opposing effects on potassium: thiazide depletes it, spironolactone retains it. Combination in resistant hypertension and ascites. In CKD, hyperkalaemia risk rises.

Symptoms

With a balanced combination, no specific symptoms. 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 2 and 4 weeks after starting, then every 3 months. With creatinine clearance below 30 mL/min, do not prescribe spironolactone. In older patients, watch for dehydration, especially in heat or acute illness.