Chlorthalidone

Key facts

• Chlorthalidone is a long-acting thiazide-like diuretic that lowers blood pressure and reduces fluid buildup by helping the kidneys pass more sodium and water into the urine.
• You take one tablet a day, usually in the morning to avoid night-time trips to the bathroom; blood pressure benefits build over several weeks.
• Chlorthalidone can lower blood potassium and sodium levels, low potassium raises the risk of dangerous heart rhythms, so periodic blood tests are needed, especially when starting or changing the dose.
• Seek urgent care for muscle cramps with weakness, an irregular heartbeat, confusion, or signs of severe dehydration such as extreme thirst and very dark urine.

What chlorthalidone treats

Chlorthalidone treats high blood pressure and fluid retention (oedema) caused by heart failure, liver disease, kidney disease, or steroid medicines. It is also used to reduce the recurrence of calcium-containing kidney stones. It does not relieve pain and will not treat an active infection.

How chlorthalidone works

Chlorthalidone acts on a segment of the kidney's filtering tubules, blocking the reabsorption of sodium and chloride back into the blood. More sodium and water then pass into the urine instead, which reduces the total fluid volume in your blood vessels. Over time this, together with a direct relaxing effect on vessel walls, lowers blood pressure.

Before you take it

• Do not take chlorthalidone if you produce no urine, or if you have a severe allergy to sulfonamide medicines, chlorthalidone is chemically related to them.
• Tell your prescriber about gout, diabetes, lupus, or kidney or liver disease, chlorthalidone can raise uric acid and blood sugar and needs caution in these conditions.
• Combined with lithium, chlorthalidone can raise lithium to toxic levels, this combination needs monitoring or dose adjustment.
• NSAIDs can reduce chlorthalidone's blood-pressure-lowering effect, and combining it with digoxin raises the risk of digoxin toxicity if potassium drops.

Side effects

Common effects include increased urination, dizziness on standing, dry mouth, and muscle cramps.

Stop and seek urgent medical care for any of these:

• Severe dehydration, extreme thirst, very dark urine, or a rapid heartbeat.
• Weakness, irregular heartbeat, or confusion from low potassium or sodium.
• Facial, lip or throat swelling, or a rash that spreads quickly.

Safety essentials

• Blood tests to check potassium, sodium and kidney function are needed before starting and periodically afterward, low potassium is the main danger and can trigger a serious heart rhythm problem.
• If you take lithium, tell your prescriber, chlorthalidone can push lithium levels into the toxic range.
• Avoid situations that cause dehydration, such as vomiting, diarrhoea, heavy sweating or excess alcohol, these worsen the drop in blood pressure and strain the kidneys.
• Report muscle cramps, weakness or a pounding heartbeat promptly, they can be early signs of a dangerous electrolyte shift.

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