Hydrochlorothiazide

Key facts

• Hydrochlorothiazide (HCTZ) is a thiazide diuretic found alone and in many combination blood-pressure tablets. It helps the kidneys remove extra salt and water.
• You usually take it once daily, in the morning, to avoid needing the bathroom overnight; it starts lowering blood pressure within a few hours.
• Long-term use raises the risk of non-melanoma skin cancer (basal cell and squamous cell carcinoma). Use sunscreen and report any new or changing skin spot.
• Seek urgent care for muscle cramps or weakness with an irregular heartbeat, or signs of severe dehydration.

What hydrochlorothiazide treats

Hydrochlorothiazide treats high blood pressure and edema, fluid buildup in the legs, ankles, or lungs from heart failure, liver disease, kidney disease, or corticosteroid use. It's also used at low doses to help prevent calcium-containing kidney stones by reducing the amount of calcium the kidneys pass into urine.

How hydrochlorothiazide works

Hydrochlorothiazide blocks a sodium-chloride transporter in the kidney's distal tubule, so more salt and water pass into the urine instead of being reabsorbed. This lowers the volume of fluid circulating in your blood vessels, which reduces blood pressure and eases swelling.

Before you take it

• Avoid hydrochlorothiazide if you cannot produce urine (anuria) or have a sulfonamide allergy, since cross-reaction is possible.
• Tell your prescriber about gout, diabetes, kidney disease, or a personal or family history of skin cancer.
• Lithium levels can rise to toxic levels when combined with hydrochlorothiazide; NSAIDs can blunt its effect; it can worsen digoxin toxicity if potassium drops.
• Routine blood tests check your sodium, potassium, and kidney function, particularly when you start or change dose.

Side effects

Common effects include passing urine more often, mild dizziness on standing, and headache.

Stop and seek urgent medical care for any of these:

• Muscle cramps, weakness, or an irregular heartbeat (signs of low potassium).
• Severe dizziness, confusion, or fainting.
• Extreme thirst, dry mouth, or very reduced urination (dehydration).
• A new or changing skin growth, or an allergic reaction with facial swelling.

Safety essentials

• The skin-cancer caution is the defining long-term risk: cumulative sun exposure while on hydrochlorothiazide increases non-melanoma skin cancer risk, so use sunscreen, protective clothing, and get any new skin lesion checked.
• Hydrochlorothiazide makes skin burn more easily in the sun (photosensitivity).
• Have your electrolytes and kidney function monitored regularly, especially if you're also on other diuretics, heart medicines, or become unwell with vomiting or diarrhea.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.