Bendroflumethiazide

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Bendroflumethiazide on Prescriptsy

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Bendroflumethiazide is a thiazide diuretic widely prescribed in the United Kingdom for the treatment of hypertension (high blood pressure) and oedema (fluid retention) associated with heart failure, hepatic cirrhosis, and nephrotic syndrome.

It is one of the most commonly used thiazide diuretics in UK general practice and has decades of clinical experience supporting its effectiveness and safety profile.

At the low 2.5 mg dose used for hypertension, bendroflumethiazide lowers blood pressure with minimal diuretic effect, while at higher doses it produces significant diuresis for the management of fluid overload.

Hypertension is the single largest modifiable risk factor for cardiovascular disease in the UK, contributing to stroke, myocardial infarction, heart failure, chronic kidney disease, and vascular dementia.

Approximately one in three adults in England has high blood pressure, and nearly half of those are undiagnosed or inadequately treated.

NICE Guideline NG136 recommends thiazide-like diuretics (including bendroflumethiazide) as one of the first-line pharmacological options for hypertension, alongside ACE inhibitors, ARBs, and calcium channel blockers.

This page provides a comprehensive clinical guide covering how bendroflumethiazide works, correct dosing, monitoring requirements, side effects, and how to obtain a prescription in the UK.

Important safety information about bendroflumethiazide

Before starting treatment, note the following key safety points about bendroflumethiazide.

• Regular blood tests are required. Your GP will check kidney function and electrolytes (especially potassium) before treatment starts and at regular intervals thereafter. Hypokalaemia (low potassium) can cause muscle weakness and potentially dangerous heart rhythm disturbances.
• Take your dose in the morning to avoid being woken at night by the need to urinate.
• Bendroflumethiazide may worsen gout, impair glucose tolerance in diabetic patients, and interact with lithium. Inform your prescriber about all medical conditions and medications.
• Do not stop taking bendroflumethiazide without consulting your GP, even if you feel well. Hypertension rarely causes symptoms, and stopping treatment abruptly may cause a rebound rise in blood pressure.

What is hypertension and why does it matter

Hypertension is defined by NICE as a sustained clinic blood pressure of 140/90 mmHg or higher, confirmed by ambulatory blood pressure monitoring (ABPM) with a daytime average of 135/85 mmHg or higher.

It is classified into stages: stage 1 (ABPM 135/85 to 149/94 mmHg), stage 2 (ABPM 150/95 mmHg or higher), and stage 3 or severe (clinic systolic 180 mmHg or higher or diastolic 120 mmHg or higher).

Most hypertension is primary (essential), meaning there is no single identifiable cause.

Risk factors include age, family history, high salt intake, obesity, physical inactivity, excessive alcohol consumption, chronic stress, and certain ethnic backgrounds (Black African and Caribbean populations have higher prevalence and earlier onset).

Secondary hypertension, caused by an underlying condition such as renal artery stenosis, primary aldosteronism, phaeochromocytoma, or Cushing syndrome, accounts for approximately 5 to 10% of cases and should be considered in patients with resistant or early-onset hypertension.

High blood pressure typically causes no symptoms until a serious complication occurs.

This is why it is often called the "silent killer." Over years, sustained elevated pressure damages the arterial endothelium, promotes atherosclerosis, and increases the workload on the heart.

The clinical consequences include ischaemic stroke and haemorrhagic stroke, myocardial infarction and coronary artery disease, heart failure (both with reduced and preserved ejection fraction), chronic kidney disease and progression to end-stage renal failure, hypertensive retinopathy, aortic aneurysm and dissection, and vascular dementia.

Each 10 mmHg reduction in systolic blood pressure reduces the risk of major cardiovascular events by approximately 20%.

How bendroflumethiazide works: mechanism of action

Bendroflumethiazide inhibits the sodium-chloride co-transporter (NCC, also known as SLC12A3) in the early distal convoluted tubule (DCT) of the nephron.

Under normal physiology, this transporter reabsorbs approximately 5 to 7% of filtered sodium from the tubular lumen back into the blood.

By blocking the NCC, bendroflumethiazide increases the excretion of sodium and chloride in the urine, with water following passively.

This natriuretic and diuretic effect is most prominent at higher doses (5 to 10 mg).

At the low 2.5 mg dose used for hypertension, the chronic blood pressure lowering effect is primarily mediated by reduced peripheral vascular resistance rather than by sustained volume depletion.

The exact vascular mechanism has not been fully elucidated but is thought to involve changes in arteriolar smooth muscle calcium handling, a reduction in vascular wall sodium content (which decreases responsiveness to vasoconstrictor hormones), and possibly a direct vasodilatory action on small resistance vessels.

The antihypertensive effect develops over 1 to 2 weeks and reaches its maximum after approximately 4 weeks of continuous treatment.

Bendroflumethiazide also increases renal calcium reabsorption (leading to mild hypercalcaemia in some patients), decreases renal uric acid excretion (potentially raising serum uric acid and precipitating gout), and can impair insulin secretion and increase insulin resistance (relevant for glucose homeostasis in patients with diabetes or pre-diabetes).

Clinical evidence and NICE guidance

Thiazide and thiazide-like diuretics have been used for the treatment of hypertension since the 1950s and are among the most extensively studied classes of antihypertensives.

The landmark ALLHAT trial demonstrated that chlorthalidone (a thiazide-like diuretic) was as effective as amlodipine and lisinopril in preventing major cardiovascular events, and superior in preventing heart failure.

While NICE NG136 has a stated preference for indapamide (a thiazide-like diuretic) over bendroflumethiazide based on clinical trial evidence, bendroflumethiazide remains very widely prescribed in UK general practice and is included as an acceptable alternative in the guideline.

NICE NG136 recommends offering antihypertensive treatment to patients with stage 1 hypertension who have target organ damage, established cardiovascular disease, renal disease, diabetes, or a 10-year cardiovascular risk of 10% or more.

For patients aged under 55 or with type 2 diabetes, first-line therapy is an ACE inhibitor or ARB.

For patients aged 55 and over, or of Black African/Caribbean family origin, a calcium channel blocker is first-line.

A thiazide-like diuretic is an alternative first-line choice at step 1, and is added at step 2 if monotherapy is insufficient.

Step 3 typically involves triple therapy (ACE inhibitor/ARB plus calcium channel blocker plus thiazide diuretic).

Dosage and administration

For hypertension, the standard dose is one 2.5 mg tablet taken once daily in the morning.

Higher doses do not provide additional blood pressure reduction but significantly increase the incidence of metabolic side effects.

The 2.5 mg dose has been established through dose-ranging studies as the optimal balance between efficacy and tolerability for blood pressure management.

For oedema, higher doses are required to produce meaningful diuresis. The initial dose is typically 5 to 10 mg daily or on alternate days, taken in the morning.

Once fluid balance is controlled, the maintenance dose is reduced to 5 mg one to three times weekly or as directed by the prescriber.

Bendroflumethiazide tablets should be swallowed whole with water and can be taken with or without food. Taking the tablet in the morning minimises nocturia.

If a morning dose is missed, it can be taken later in the day, but taking it in the evening should be avoided due to the diuretic effect.

Monitoring requirements

Before starting bendroflumethiazide, baseline blood tests should include renal function (creatinine, eGFR), serum electrolytes (potassium, sodium, calcium, magnesium), blood glucose or HbA1c, and uric acid in patients with a history of gout.

Follow-up blood tests should be performed at 4 to 6 weeks after initiation, and then at least annually if results are stable.

More frequent monitoring is required if the patient is taking concurrent medications that affect potassium (ACE inhibitors, ARBs, potassium-sparing diuretics, NSAIDs), if renal function is borderline, or if the patient is elderly.

Side effects of bendroflumethiazide

Metabolic side effects

The metabolic effects of thiazide diuretics are the most clinically important considerations. Hypokalaemia is the most common and significant electrolyte disturbance, occurring more frequently at higher doses.

Serum potassium below 3.5 mmol/L requires intervention, which may include dietary potassium supplementation (bananas, oranges, potatoes), addition of a potassium-sparing agent (such as amiloride or spironolactone), or switching to an alternative antihypertensive.

Severe hypokalaemia (below 3.0 mmol/L) increases the risk of cardiac arrhythmias and requires urgent correction.

Hyponatraemia (low sodium) is more common in elderly patients, particularly in warm weather, and can present with confusion, lethargy, nausea, and in severe cases seizures.

Hyperuricaemia is caused by reduced renal uric acid excretion and can precipitate acute gout in susceptible individuals.

Patients with a history of gout should be counselled about this risk.

Impaired glucose tolerance and hyperglycaemia are dose-dependent effects that may be clinically relevant in patients with type 2 diabetes or pre-diabetes.

Blood glucose should be monitored more frequently in these patients. Mild hypercalcaemia occurs because thiazides increase renal tubular calcium reabsorption.

This is usually clinically insignificant but should be noted in patients with suspected hyperparathyroidism.

Cardiovascular side effects

Postural hypotension (dizziness or light-headedness on standing) is common, particularly in the first days of treatment and in elderly patients.

This occurs because the combination of volume depletion and vasodilation reduces the body's ability to compensate for the gravitational shift in blood distribution when standing.

Standing up slowly, staying well hydrated, and avoiding prolonged standing can help manage this effect.

Other side effects

Nausea, mild gastrointestinal disturbance, headache, and lethargy may occur. Skin rashes and photosensitivity (increased sunburn susceptibility) have been reported.

Erectile dysfunction is an uncommon but recognised side effect of thiazide diuretics. If this occurs, discuss alternative antihypertensive options with your GP.

Blood dyscrasias (thrombocytopenia, leucopenia, agranulocytosis) are rare but serious. Unexplained bruising, bleeding, sore throat, or fever during treatment should prompt urgent blood count testing.

When to seek medical advice

Contact your GP or call NHS 111 if you experience persistent muscle cramps (possible hypokalaemia), confusion or unusual drowsiness (possible hyponatraemia), severe joint pain (possible gout), significant dizziness or fainting, skin rash or increased sensitivity to sunlight, or unusual bruising or bleeding.

Seek emergency care (call 999 or attend A&E) if you experience palpitations, chest pain, or signs of severe allergic reaction.

Report any suspected adverse reactions via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk .

Drug interactions

Bendroflumethiazide interacts with a number of commonly prescribed medicines. Important interactions include the following.

• Lithium: Thiazide diuretics reduce renal lithium clearance, causing lithium accumulation and toxicity. Concomitant use should be avoided where possible. If unavoidable, lithium levels must be monitored frequently and the dose adjusted.
• NSAIDs (ibuprofen, naproxen, diclofenac): Non-steroidal anti-inflammatory drugs reduce the antihypertensive effect of thiazides and increase the risk of renal impairment. Use the lowest effective dose for the shortest duration.
• Digoxin: Hypokalaemia caused by thiazides increases the risk of digoxin toxicity and cardiac arrhythmias. Serum potassium must be maintained within the normal range.
• Corticosteroids: Concurrent use increases potassium depletion.
• ACE inhibitors and ARBs: These counteract potassium loss (protective) but may increase the risk of hypotension and renal impairment, particularly on initiation.
• Antidiabetic medicines: Thiazides may impair glucose control, requiring dose adjustment of insulin or oral hypoglycaemics.

How to get a bendroflumethiazide prescription in the UK

Bendroflumethiazide is a prescription-only medicine (POM) in the UK.

It is usually prescribed by your GP following a comprehensive cardiovascular risk assessment, including blood pressure measurement (ideally confirmed by ABPM), blood tests, and a review of your cardiovascular risk factors.

Repeat prescriptions are managed through your GP practice.

Authorised online prescribers registered with the GPhC may prescribe bendroflumethiazide following a structured clinical consultation, provided that an appropriate assessment has been completed and regular monitoring is in place.

All UK prescriptions are dispensed by registered pharmacies.

The standard NHS prescription charge in England is 9.90 pounds per item; prescriptions are free in Scotland, Wales, and Northern Ireland.

Lifestyle measures alongside medication

NICE recommends lifestyle modifications alongside pharmacological treatment for hypertension.

These are effective both as adjuncts to medication and, in some patients with stage 1 hypertension and low cardiovascular risk, as an alternative to drug therapy.

Key measures include reducing dietary salt intake to less than 6 g per day, eating a diet rich in fruit, vegetables, and wholegrains with reduced saturated fat (the DASH diet pattern), regular aerobic exercise (at least 150 minutes of moderate-intensity activity per week), maintaining a healthy body weight (BMI 20 to 25 kg/m2), limiting alcohol intake to 14 units per week or fewer, stopping smoking, and reducing caffeine intake.

Stress management techniques and adequate sleep also contribute to blood pressure control.

When to seek urgent medical advice

Hypertension is usually asymptomatic, and the primary risk is long-term cardiovascular damage from inadequate treatment.

However, severe hypertension (blood pressure above 180/120 mmHg) can occasionally present acutely with symptoms such as severe headache, visual disturbance, chest pain, breathlessness, or neurological symptoms.

This is a medical emergency (hypertensive crisis) requiring immediate assessment. Call 999 or attend A&E.

For non-urgent concerns about your blood pressure or medication, contact your GP or NHS 111.

Report any suspected adverse reactions via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk .

Sources

• Bendroflumethiazide 2.5 mg Tablets, Summary of Product Characteristics (EMC)
• Bendroflumethiazide, British National Formulary (BNF)
• NICE NG136: Hypertension in adults: diagnosis and management
• NICE CKS: Hypertension
• High blood pressure (hypertension), NHS
• MHRA Yellow Card Scheme