Thiazide diuretic Pregnancy: Not recommended in pregnancy. Hydrochlorothiazide crosses the placenta; second/third-trimester use may compromise foeto-placental perfusion and cause neonatal icterus, electrolyte disturbance, bone-marrow depression and thrombocytopenia. Do not use for gestational oedema/hypertension or pre-eclampsia. Excreted in breast milk in small amounts; breast-feeding not recommended.

Hydrochlorothiazide

Hydrochlorothiazide is a thiazide diuretic used in the management of hypertension and oedema.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Hydrochlorothiazide monotherapy is effective at 12.5 mg to 50 mg per day (per US labelling)

Route: Oral

Frequency: Once daily

CAUTION — no standalone hydrochlorothiazide SPC was in the bundle. The eMC provider is a fixed-dose combination (Co-amilozide 2.5 mg/25 mg tablet = amiloride 2.5 mg + hydrochlorothiazide 25 mg), so its doses are expressed as combination-tablet counts, not standalone hydrochlorothiazide. The standalone figure quoted here ('hydrochlorothiazide monotherapy is effective in doses of 12.5 mg to 50 mg per day') is from US labelling of a lisinopril/hydrochlorothiazide combination product. eMC Co-amilozide posology (combination tablet): Hypertension — initially one 2.5/25 mg tablet once daily, may increase to two tablets once daily or in divided doses. Congestive heart failure — initially one tablet daily, adjusted, not exceeding four tablets daily. Hepatic cirrhosis with ascites — start low (up to two tablets daily, increase gradually), not exceeding four tablets daily. Dosage governed by rate of weight loss (target 0.5–1.0 kg/day) and serum electrolytes. Not recommended for children under 18 years (safety/efficacy not established). Particular caution in the elderly (electrolyte imbalance). Verify the intended standalone hydrochlorothiazide dose against a UK monotherapy SPC.

Dose adjustments

Renal

Thiazide diuretics become ineffective when creatinine clearance falls below 30 ml/min. Contraindicated in severe renal impairment; the Co-amilozide combination risks rapid hyperkalaemia in renal impairment. US labelling: for creatinine clearance ≤30 mL/min/1.73m², loop diuretics preferred to thiazides.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to hydrochlorothiazide or other sulfonamide-derived drugs (and, for the Co-amilozide combination, to amiloride)
Anuria; severe / acute / progressive renal impairment; severe renal disease
Hyperkalaemia (plasma potassium >5.5 mmol/l); concomitant potassium-conserving diuretics, potassium supplements or potassium-rich food (except closely monitored refractory hypokalaemia)
Concomitant spironolactone or triamterene
Severe hepatic failure; precoma associated with hepatic cirrhosis
Addison's disease; hypercalcaemia; diabetes mellitus / diabetic nephropathy
Concurrent lithium therapy

Side effects

Electrolyte imbalance — hyponatraemia, hypokalaemia (thiazide); hyperkalaemia (amiloride component)
Dizziness, headache, orthostatic hypotension
Nausea, vomiting, diarrhoea, constipation, abdominal pain
Muscle cramps, leg ache
Rash, pruritus; impotence (reversible)

Interactions

Lithium — concurrent therapy contraindicated (thiazides reduce lithium clearance)
Potassium-conserving diuretics (spironolactone, triamterene), potassium supplements / potassium-rich diet — risk of hyperkalaemia with the amiloride component; contraindicated
Cardiac glycosides (digitalis) — thiazide-induced hypokalaemia can precipitate digitalis toxicity
Other antihypertensives — additive blood-pressure lowering; may need dose reduction

Clinical monograph

How it works

It inhibits the sodium-chloride co-transporter in the distal convoluted tubule, increasing urinary sodium and water loss and, with chronic use, lowering peripheral vascular resistance.

Prescribing in practice

It can cause electrolyte disturbances, particularly hypokalaemia and hyponatraemia, so electrolytes should be checked and patients monitored, especially in the elderly.
An MHRA alert has highlighted a cumulative-dose-related risk of non-melanoma skin cancer, and patients should be advised on sun protection and to report new or changing skin lesions.
It may unmask or worsen gout and can affect glucose and lipid control; use with caution in renal or hepatic impairment.

Monitoring

Monitor serum electrolytes, renal function and blood pressure, particularly after initiation and in older or at-risk patients.

Counselling the patient

Use sun protection and report any new, changing or non-healing skin lesions.
Report symptoms of low electrolytes such as marked weakness, cramps, confusion or dizziness.
Take the dose in the morning to reduce night-time urination.

Evidence & guidelines

An MHRA Drug Safety Update warns of an increased risk of non-melanoma skin cancer with cumulative hydrochlorothiazide exposure, and thiazides are recommended in NICE hypertension guidance.

Reference: NICE NG136; MHRA Drug Safety Update; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

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