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Thiazide diuretic Pregnancy: Use during pregnancy only if clearly needed: there are no adequate and well-controlled studies in pregnant women (animal studies in mice and rats showed no evidence of fetal harm). Thiazides cross the placental barrier and appear in cord blood; there is a risk of fetal or neonatal jaundice, thrombocytopenia and possibly other adverse reactions that have occurred in adults. Routine use of diuretics during normal pregnancy is inappropriate and exposes mother and fetus to unnecessary hazard; diuretics do not prevent toxaemia of pregnancy and there is no satisfactory evidence that they are useful in its treatment. Thiazides are indicated in pregnancy when oedema is due to pathologic causes. Nursing mothers: thiazides are excreted in breast milk; because of the potential for serious adverse reactions in nursing infants, a decision should be made whether to discontinue nursing or to discontinue hydrochlorothiazide.

Hydrochlorothiazide

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

Adult dose

Dose: Hypertension (sole agent or added to other antihypertensives): usual initial dose 25 mg once daily as a single dose; may be increased to 50 mg daily, given as a single dose or in two divided doses. Oedema (congestive heart failure, hepatic cirrhosis, corticosteroid and estrogen therapy; also oedema due to renal dysfunction): usual adult dosage 25 to 100 mg daily as a single or divided dose.
Route: Oral
Frequency: Once daily (hypertension: single daily dose, or up to 50 mg/day as two divided doses; oedema: single or divided daily dose)
Single-ingredient hydrochlorothiazide tablets (not a combination product). Label wording: 'Therapy should be individualized according to patient response. Use the smallest dosage necessary to achieve the required response.' Hypertension: 'The usual initial dose in adults is 25 mg daily given as a single dose. The dose may be increased to 50 mg daily, given as a single or two divided doses. Doses above 50 mg are often associated with marked reductions in serum potassium. Patients usually do not require doses in excess of 50 mg of hydrochlorothiazide daily when used concomitantly with other antihypertensive agents.' Oedema: 'The usual adult dosage is 25 to 100 mg daily as a single or divided dose. Many patients with edema respond to intermittent therapy, i.e., administration on alternate days or on three to five days each week. With an intermittent schedule, excessive response and the resulting undesirable electrolyte imbalance are less likely to occur.' The label prints no separate maximum daily dose, so none is stored. No UK SPC for hydrochlorothiazide is listed on eMC — this dose is from US prescribing information (Hydrochlorothiazide Tablets USP, REMEDYREPACK INC.) and must be verified against UK practice.

Paediatric dose

Route: Oral
Frequency: Single dose or two divided doses per day
Max: Not to exceed 37.5 mg per day in infants up to 2 years of age, or 100 mg per day in children 2 to 12 years of age
Infants and children, for diuresis and for control of hypertension, label wording: 'The usual pediatric dosage is 0.5 to 1 mg per pound (1 to 2 mg/kg) per day in single or two divided doses, not to exceed 37.5 mg per day in infants up to 2 years of age or 100 mg per day in children 2 to 12 years of age. In infants less than 6 months of age, doses up to 1.5 mg per pound (3 mg/kg) per day in two divided doses may be required.' dosePerKg is left null because the label gives a RANGE (1 to 2 mg/kg/day) and a separate infant allowance (up to 3 mg/kg/day in two divided doses for infants under 6 months). The label notes there are no well-controlled clinical trials in paediatric patients; dosing information is supported by empiric use and published literature on paediatric hypertension. No label ceiling is given for children over 12 years. US labelling — verify against a children's formulary.

Dose adjustments

Renal

No renal dose figure is given. Use with caution in severe renal disease; in patients with renal disease thiazides may precipitate azotaemia, and cumulative effects may develop in impaired renal function. If progressive renal impairment becomes evident, consider withholding or discontinuing diuretic therapy. Contraindicated in anuria.

Hepatic

No hepatic dose figure is given. Use with caution in impaired hepatic function or progressive liver disease, since minor alterations of fluid and electrolyte balance may precipitate hepatic coma.

Dose independently re-sourced from US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — clinician-reviewed. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

  • Anuria
  • Hypersensitivity to this product or to other sulfonamide-derived drugs

Side effects

  • Electrolyte imbalance (hyponatraemia, hypochloraemic alkalosis, hypokalaemia); hyperuricaemia (gout may be precipitated); hyperglycaemia and glycosuria
  • Hypotension including orthostatic hypotension (may be aggravated by alcohol, barbiturates, narcotics or antihypertensive drugs); weakness
  • Gastrointestinal: pancreatitis, jaundice (intrahepatic cholestatic), diarrhoea, vomiting, sialadenitis, cramping, constipation, gastric irritation, nausea, anorexia
  • Haematological: aplastic anaemia, agranulocytosis, leukopenia, haemolytic anaemia, thrombocytopenia
  • Hypersensitivity: anaphylactic reactions, necrotizing angiitis (vasculitis), respiratory distress including pneumonitis and pulmonary oedema, photosensitivity, fever, urticaria, rash, purpura
  • Dizziness, vertigo, headache, paraesthesias, restlessness; muscle spasm; impotence
  • Renal failure, renal dysfunction, interstitial nephritis
  • Severe skin reactions: erythema multiforme including Stevens-Johnson syndrome, exfoliative dermatitis including toxic epidermal necrolysis; alopecia
  • Transient blurred vision, xanthopsia; acute transient myopia and acute angle-closure glaucoma (idiosyncratic reaction in a sulfonamide, typically within hours to weeks of starting)
  • Increased risk of non-melanoma skin cancer, predominantly squamous cell carcinoma, with large cumulative doses

Interactions

  • Alcohol, barbiturates or narcotics - potentiation of orthostatic hypotension
  • Antidiabetic drugs (oral agents and insulin) - dosage adjustment of the antidiabetic drug may be required
  • Other antihypertensive drugs - additive effect or potentiation
  • Cholestyramine and colestipol resins - absorption of hydrochlorothiazide impaired (single doses reduce absorption by up to 85% and 43% respectively)
  • Corticosteroids, ACTH - intensified electrolyte depletion, particularly hypokalaemia
  • Pressor amines (e.g. norepinephrine) - possible decreased response, not sufficient to preclude their use
  • Non-depolarising skeletal muscle relaxants (e.g. tubocurarine) - possible increased responsiveness to the muscle relaxant
  • Lithium - generally should not be given with diuretics; diuretics reduce renal clearance of lithium and add a high risk of lithium toxicity
  • Non-steroidal anti-inflammatory drugs - can reduce the diuretic, natriuretic and antihypertensive effects; observe closely
  • Digitalis - hypokalaemia may sensitise or exaggerate the response of the heart to the toxic effects of digitalis

Monitoring

  • Observe all patients for evidence of fluid or electrolyte imbalance (hyponatraemia, hypochloraemic alkalosis, hypokalaemia); periodic determination of serum electrolytes at appropriate intervals
  • Serum and urine electrolytes are particularly important when the patient is vomiting excessively or receiving parenteral fluids
  • Glycaemic control in diabetic patients (insulin or oral hypoglycaemic dose adjustment may be needed; latent diabetes may become manifest)
  • Hyperuricaemia / acute gout; increases in cholesterol and triglyceride levels
  • Serum calcium - marked hypercalcaemia may indicate hidden hyperparathyroidism; discontinue thiazides before parathyroid function tests
  • Renal function - if progressive renal impairment becomes evident, consider withholding or discontinuing diuretic therapy
  • Visual symptoms (acute onset of decreased visual acuity or ocular pain) - discontinue promptly if acute myopia or angle-closure glaucoma is suspected

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).

Related

Curated clinical cross-links plus same-class fallbacks.

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