Hydrochlorothiazide
Hydrochlorothiazide is a thiazide diuretic widely used as first-line therapy for hypertension and as adjunctive treatment for oedema. It is on the WHO Essential Medicines List and is available in low doses (12.5–25mg) as monotherapy or in fixed combinations.
- Chemical formula
- C7H8ClN3O4S2
- CAS number
- 58-93-5
- ATC code
- C03AA03
- Molecular weight
- 297.74 g/mol
- Drug class
- Thiazide diuretic
- Also known as
- HCTZ, Microzide, HydroDiuril
What is it?
Hydrochlorothiazide (HCTZ) is one of the oldest and most prescribed thiazide diuretics, in clinical use since 1959. It is on the WHO Essential Medicines List and is the most widely used antihypertensive in many countries due to low cost, oral once-daily dosing, and long safety record. It is available as monotherapy (Microzide, HydroDiuril) and in many fixed combinations with ACE inhibitors, ARBs, beta-blockers and potassium-sparing diuretics.
Mechanism of action
Hydrochlorothiazide blocks the sodium-chloride symporter in the distal convoluted tubule of the kidney, reducing sodium reabsorption and producing modest diuresis. The blood pressure-lowering effect develops over 1–2 weeks and persists with continued dosing through both diuretic and direct vasodilatory mechanisms. The diuretic effect is moderate compared with loop diuretics like furosemide.
Pharmacokinetics
Hydrochlorothiazide is well absorbed after oral administration with bioavailability of 60–80%. Onset of diuresis is within 2 hours, peak effect at 4–6 hours, and duration approximately 12 hours. The terminal half-life is 6–15 hours, supporting once-daily dosing. The drug is not metabolised significantly and is excreted unchanged in urine, making renal function a key determinant of effect.
Indications
Hydrochlorothiazide is approved for hypertension (alone or in combination), oedema in heart failure, hepatic cirrhosis or nephrotic syndrome, and certain renal conditions. It is generally considered first-line for hypertension in older adults and Black patients, often in combination with an ACE inhibitor or ARB. According to current hypertension guidelines, low doses (12.5–25mg) are preferred over the older 50mg dose because they preserve metabolic profile while retaining most of the blood pressure benefit.
Safety profile
Common adverse effects include hypokalaemia, hyponatraemia, hyperuricaemia (with gout flares), hyperglycaemia, dyslipidaemia and orthostatic hypotension. Most are dose-related and minimised by using low doses (12.5–25mg). Sulfa allergy is a relative contraindication, although true cross-reactivity with sulfonamide antibiotics is uncommon. Serum potassium, sodium, glucose and uric acid require periodic monitoring.
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Frequently asked questions
Why is hydrochlorothiazide first-line for hypertension? ▾
Decades of evidence show meaningful blood pressure reduction and reduced cardiovascular events with HCTZ. It is inexpensive, well-tolerated at low doses, and combines well with ACE inhibitors and ARBs. According to current guidelines, it is one of the four first-line classes for adult hypertension along with ACE inhibitors, ARBs and calcium channel blockers.
What lab tests are needed on hydrochlorothiazide? ▾
Serum potassium, sodium, glucose, uric acid and renal function are checked at baseline and periodically — typically at 1–2 months after initiation and annually thereafter. Hypokalaemia is the most common abnormality; potassium-rich diet, potassium-sparing diuretics or ACE inhibitor combination usually manages it without supplementation.
Can I take hydrochlorothiazide if I have gout? ▾
Hydrochlorothiazide raises serum uric acid and can precipitate gout flares. In patients with gout history, the prescriber may use lower HCTZ doses, alternative antihypertensives, or add allopurinol to control uric acid. According to current guidelines, gout is not an absolute contraindication but warrants individual assessment.
The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.