Bendroflumethiazide

Bendroflumethiazide is a thiazide diuretic (commonly called a "water tablet") used in the management of hypertension (high blood pressure) and oedema (fluid retention). It has been used in clinical practice for decades and remains on the World Health Organisation Essential Medicines List as a first-line antihypertensive agent. In the UK, bendroflumethiazide is often prescribed as part of a broader cardiovascular risk reduction strategy, sometimes in combination with other antihypertensive classes such as ACE inhibitors, angiotensin receptor blockers, or calcium channel blockers.

How Bendroflumethiazide Works

Bendroflumethiazide acts on the distal convoluted tubule of the nephron in the kidney, where it inhibits the sodium-chloride co-transporter. This reduces the reabsorption of sodium and water from the urine back into the bloodstream, resulting in increased urine output and a reduction in circulating blood volume. In hypertension, this volume reduction lowers cardiac output and subsequently reduces blood pressure. With continued use, there is also a direct vasodilatory effect on peripheral blood vessels that contributes to the sustained blood pressure-lowering action.

Role in Managing Oedema

Bendroflumethiazide is also used to manage oedema associated with heart failure, liver cirrhosis, and nephrotic syndrome, where the body retains excess fluid in the tissues. By promoting the excretion of sodium and water through the kidneys, it reduces swelling in the ankles, legs, and abdomen. For most patients with hypertension, a low dose of 2.5 mg once daily is sufficient, while higher doses of 5 to 10 mg may be needed when treating oedema.

Monitoring Requirements

Regular blood tests are necessary for patients taking bendroflumethiazide to monitor electrolyte levels, particularly sodium, potassium, and urea, as the drug promotes the loss of these electrolytes through the urine. Low potassium (hypokalaemia) is the most clinically important electrolyte disturbance, as it can affect cardiac rhythm. Blood glucose and uric acid levels should also be monitored.

How to Take Bendroflumethiazide

Take bendroflumethiazide as a tablet once daily, in the morning. Taking it in the morning is strongly recommended because the diuretic effect peaks within two to six hours, and an evening dose can cause disruptive overnight urination. Take with a glass of water, with or without food.

For high blood pressure, treatment is long term. Take your tablet every day at the same time, even when your blood pressure is well controlled and you feel well. Do not stop suddenly without medical advice, as blood pressure can rise sharply.

Diet and Missed Doses

Maintain adequate fluid intake and eat potassium-rich foods (bananas, oranges, tomatoes, potatoes) to help offset potassium losses. If potassium remains low despite dietary measures, your doctor may add a supplement. If you miss a dose, take it as soon as you remember on the same day.

For hypertension, the recommended starting dose is 2.5 mg once daily in the morning. This dose is sufficient to produce the full antihypertensive effect in most patients; higher doses do not provide significantly greater blood pressure reduction but do increase the risk of electrolyte disturbances.

For the treatment of oedema, the usual dose is 5 to 10 mg once daily or on alternate days, adjusted according to clinical response and the degree of fluid retention. Once the oedema is controlled, the dose is often reduced to the minimum required for maintenance.

Elderly patients may be more susceptible to electrolyte disturbances and postural hypotension; the lowest effective dose should be used, and electrolytes should be monitored closely. Bendroflumethiazide is not recommended in severe renal impairment (eGFR below 30 mL/min/1.73m2), as it is ineffective when glomerular filtration is severely reduced. In moderate renal impairment, it should be used with caution and with regular monitoring of renal function and electrolytes. It is generally avoided in severe hepatic impairment due to the risk of electrolyte disturbances precipitating hepatic encephalopathy.

Bendroflumethiazide is generally well tolerated, but electrolyte disturbances are an important concern with regular use.

Common Side Effects

The following side effects are frequently reported:

• Hypokalaemia (low potassium): muscle cramps, weakness, fatigue, abnormal heart rhythms
• Hyponatraemia (low sodium): headache, confusion, nausea
• Increased urination, particularly in the first few weeks of treatment
• Postural hypotension (dizziness when standing up quickly)
• Elevated blood glucose levels (may worsen or unmask diabetes)
• Raised uric acid levels (may precipitate gout in predisposed individuals)
• Mild gastrointestinal symptoms: nausea, loss of appetite, stomach discomfort

Serious Side Effects

The following require prompt medical assessment:

• Severe electrolyte imbalance: signs of serious hypokalaemia include palpitations, muscle paralysis, or a severely abnormal heart rhythm, requires urgent medical review
• Severe hyponatraemia: confusion, seizures, or loss of consciousness due to very low sodium levels
• Acute pancreatitis: severe upper abdominal pain radiating to the back (rare but reported)
• Severe skin reactions: Stevens-Johnson syndrome or toxic epidermal necrolysis (extremely rare)
• Agranulocytosis or thrombocytopaenia: unusual bruising, bleeding, or frequent infections, indicating bone marrow suppression

Electrolyte Monitoring

Regular blood tests to measure electrolytes, particularly potassium, sodium, magnesium, and renal function (urea and creatinine), are essential during bendroflumethiazide treatment. The frequency of monitoring should be higher when starting treatment, when the dose is changed, in elderly patients, and in those with renal impairment or those taking other medicines that affect electrolytes (such as ACE inhibitors or digoxin).

Interactions with Other Medicines

Bendroflumethiazide has several clinically important drug interactions. It enhances the effect of other antihypertensive drugs, which can lead to an excessive fall in blood pressure. It can increase the toxicity of lithium by reducing renal lithium excretion. Concurrent use with non-steroidal anti-inflammatory drugs (NSAIDs) may blunt the antihypertensive effect and worsen renal function. Corticosteroids and carbenoxolone increase potassium loss when used alongside thiazides.

Diabetes, Gout, and Pregnancy

Bendroflumethiazide can impair glucose tolerance and may worsen glycaemic control in patients with diabetes. Blood glucose monitoring should be intensified when starting treatment. Patients with a history of gout should be monitored for increased uric acid levels and gout attacks. Bendroflumethiazide is generally avoided in pregnancy, particularly in the first trimester, because of potential effects on foetal electrolyte balance and the risk of neonatal hypoglycaemia and jaundice.

Bendroflumethiazide must not be used in the following circumstances:

• Known hypersensitivity to bendroflumethiazide, other thiazide diuretics, or sulfonamide-derived drugs
• Severe renal impairment (eGFR below 30 mL/min/1.73m2) -- ineffective and risk of worsening renal failure
• Severe hepatic impairment or active hepatic encephalopathy
• Pre-existing hypokalaemia or hyponatraemia
• Refractory hypercalcaemia (thiazides reduce urinary calcium excretion)
• Addison's disease (adrenal insufficiency)
• Pregnancy (especially first trimester)
• Simultaneous use with lithium without careful monitoring (risk of lithium toxicity)