Hydrochlorothiazide is a thiazide diuretic — a “water tablet”. It is used for high blood pressure and for fluid retention caused by heart, liver or kidney conditions. It makes the kidneys excrete more salt and water. It appears in many South African products, both alone and combined with other blood pressure medicines.
What is Hydrochlorothiazide?
Hydrochlorothiazide is a thiazide diuretic, generally Schedule 3 in South Africa. It is one of the most widely used blood pressure ingredients in the country and appears in a great many products:
| Product | Contains |
|---|---|
| Ridaq | Hydrochlorothiazide alone (12.5 mg, 25 mg) |
| Gulf Hydrochlorothiazide | Hydrochlorothiazide 25 mg |
| Adco-Retic, Amiloretic, Co-Amilozide | Hydrochlorothiazide 50 mg + amiloride 5 mg |
| Renezide | Hydrochlorothiazide 25 mg + triamterene 50 mg |
| Adco-Zetomax Co | Hydrochlorothiazide + lisinopril |
| Bilocor Co | Hydrochlorothiazide + bisoprolol |
Check what your tablet actually contains. Hydrochlorothiazide is so often combined with something else that many people take it without realising, and the additional ingredient changes the cautions considerably.
It works on a specific part of the kidney — the distal convoluted tubule — where it blocks a transporter that would otherwise reabsorb sodium and chloride back into the blood. More sodium stays in the urine, water follows it, and the volume of fluid in the circulation falls.
But the long-term effect on blood pressure is not mainly about fluid. In the first weeks, reduced blood volume does the work. Over months, blood volume largely returns to normal while blood pressure stays down, because the blood vessels themselves relax. This is why hydrochlorothiazide keeps working long after the diuretic effect has faded, and why it takes a few weeks to show its full benefit.
A practical consequence: you will pass more urine in the first days or weeks, and that settles. Many people expect this to continue and worry when it stops — it is normal.
What is Hydrochlorothiazide commonly used for?
- High blood pressure (hypertension) — the main use, often as part of a combination.
- Oedema (fluid retention) from heart failure, liver disease or kidney conditions.
- Heart failure, to reduce fluid overload and breathlessness.
- Nephrogenic diabetes insipidus, a condition of excessive urination where, counter-intuitively, a diuretic helps.
- Preventing recurrent calcium kidney stones, because thiazides reduce calcium in the urine.
High blood pressure has no symptoms in almost everyone who has it. You will likely feel no different on this medicine, and feeling well is not a reason to stop. It treats a risk — of stroke, heart attack, kidney damage and heart failure — rather than a sensation.
Thiazides are among the best-studied blood pressure treatments, with strong evidence for reducing strokes and heart attacks, and they are inexpensive. They are frequently combined with an ACE inhibitor, ARB or calcium channel blocker — combining mechanisms at moderate doses works better, with fewer side effects, than pushing one medicine to its maximum.
How Hydrochlorothiazide fits into this medicine category
| Group | Examples | What it does |
|---|---|---|
| Thiazide diuretics | Hydrochlorothiazide, indapamide | Blood pressure, mild fluid retention |
| Loop diuretics | Furosemide | Stronger, faster; heart failure and significant oedema |
| Potassium-sparing diuretics | Amiloride, spironolactone | Weak alone; used to protect potassium |
| ACE inhibitors | Enalapril, lisinopril | Relax vessels; protect heart and kidneys |
| Calcium channel blockers | Amlodipine | Relax and widen blood vessels |
| Beta-blockers | Bisoprolol, carvedilol | Slow the heart, reduce its workload |
Thiazides and loop diuretics are not interchangeable. Furosemide is far more powerful and better for significant fluid overload; thiazides are better for long-term blood pressure control. See the blood pressure, heart and circulation medicines hub.
What to check before using Hydrochlorothiazide
Take it in the morning. Taken later, it will wake you at night to pass urine. If you take it twice daily, make the second dose early afternoon.
Expect blood tests. Potassium, sodium, kidney function, glucose and sometimes uric acid are checked periodically. These are not a formality — salt imbalance is the main risk of this medicine, and it is usually silent until it is significant.
Tell your doctor or pharmacist if you:
- Have gout, or raised uric acid. Thiazides reduce uric acid excretion and can precipitate an attack.
- Have diabetes, or are at risk. Thiazides can raise blood sugar slightly.
- Have kidney or liver disease.
- Have low potassium or sodium, or are prone to them.
- Have lupus, which can be aggravated.
- Have a sulfonamide allergy.
- Are pregnant or breastfeeding.
- Are elderly, where low sodium and dehydration are more likely.
- Take digoxin — low potassium makes digoxin toxicity more likely.
- Take lithium, whose levels rise dangerously with thiazides.
- Take anti-inflammatories such as ibuprofen. NSAIDs raise blood pressure, blunt diuretics, and combined with an ACE inhibitor or ARB can cause acute kidney injury.
- Take potassium supplements or other diuretics.
- Take medicines affecting heart rhythm.
Be careful if you become unwell with vomiting, diarrhoea or fever, or in very hot weather. Dehydration on a diuretic can affect the kidneys quickly. Ask your doctor whether to pause it temporarily — many will advise “sick day rules” for exactly this.
Possible cautions and when to ask a pharmacist or doctor
Common effects are passing more urine early on, dizziness when standing, tiredness, muscle cramps, and increased sensitivity to sunlight. It can also cause erectile difficulty, which is worth mentioning rather than tolerating, since alternatives exist.
Seek medical advice if you develop:
- Muscle cramps or weakness, unusual tiredness, palpitations or an irregular heartbeat — possible low potassium.
- Confusion, marked drowsiness, headache, nausea or a seizure — possible low sodium, more common in older people and in hot weather.
- A painful, hot, swollen joint, often the big toe — a gout attack.
- Marked thirst, frequent urination and blurred vision — possible raised blood sugar.
- Severe dizziness or fainting.
- Unusual or severe sunburn, or a rash on sun-exposed skin.
- A rash, blistering or peeling skin.
- Much less urine than usual.
- Sudden breathlessness or reduced vision with eye pain — rare but urgent.
Use sun protection. Photosensitivity is a genuine and often overlooked effect, and thiazides have been associated with a small increased risk of certain skin cancers with long-term use — another reason to be sun-sensible rather than alarmed.
Stand up slowly, particularly at first, and take care with alcohol, which lowers blood pressure further.
Tell your doctor if you become pregnant, and check before starting any new medicine — especially over-the-counter anti-inflammatories.
Related medicine guides
- Browse Blood Pressure, Heart & Circulation Medicines guides
- Read: Ciplavasc used for
- Read: Ridaq used for
Frequently asked questions
What is hydrochlorothiazide used for? High blood pressure and fluid retention from heart, liver or kidney conditions. It is a thiazide diuretic, or water tablet.
What is hydrochlorothiazide 25 mg used for? A common strength for blood pressure, alone or combined with another medicine. Lower strengths such as 12.5 mg are also widely used for blood pressure.
What is hydrochlorothiazide 12.5 mg used for? Usually blood pressure, often in a combination tablet. Lower doses control blood pressure effectively with less effect on potassium and blood sugar.
When should I take it? In the morning, so increased urination does not disturb your sleep.
Why do I need blood tests? To check potassium, sodium and kidney function. Salt imbalance is the main risk and is usually silent until significant.
Will I keep passing more urine? Usually only for the first days or weeks. The blood pressure benefit continues after that effect settles.
Can I take ibuprofen with it? Ask a pharmacist. NSAIDs raise blood pressure, reduce the medicine’s effect, and combined with an ACE inhibitor or ARB can harm the kidneys.
Does it affect gout or diabetes? It can raise uric acid and trigger gout, and slightly raise blood sugar. Tell your doctor if either applies to you.