Thiazide Diuretics

What are thiazide diuretics and what are they used for?

A diuretic is a medicine which increases the amount of water that you pass out from your kidneys. (A diuretic causes a 'diuresis' - an increase in urine.) So, they are sometimes called 'water tablets'. There are different types of diuretics which work in different ways.

Thiazide diuretics are one type of diuretic. The most commonly used thiazide is called bendroflumethiazide (bendrofluazide). Other thiazide diuretics include chlortalidone (chlorthalidone), benzthiazide, clopamide, cyclopenthiazide, hydrochlorothiazide, xipamide, indapamide, hydroflumethiazide and metolazone. Each comes in different brand names.

Thiazide diuretics are a common treatment for high blood pressure. They are also used to clear fluid from the body in conditions where your body accumulates too much fluid such as heart failure. (But, a type of diuretic called a 'loop diuretic' is more commonly used to treat heart failure.)

How do thiazide diuretics work?

One of their effects is to make the kidneys pass out more fluid. They do this by interfering with the transport of salt and water across certain cells in the kidneys. Thiazide diuretics tend to have only a weak action on the kidneys so you dont notice a great increase of urine if you take these (compared to loop diuretics). They also have an effect to widen (dilate) blood vessels. A combination of these two effects reduces the blood pressure.

What are the possible side-effects of thiazide diuretics?

Side-effects are uncommon as the dose needed to lower blood pressure is low. The leaflet which comes in the tablet package provides a full list of possible side-effects. These include:

• A possible increase in your blood sugar level. Some people with diabetes may need more treatment to keep the blood sugar level normal.
• A possible increase the level of uric acid. So, if you have gout you may have more gout attacks. A first attack of gout is sometimes triggered by taking a diuretic.
• The salt balance in the bloodstream is sometimes upset which can cause a low blood level of potassium, sodium, and magnesium, and a high level of calcium level. These effects may cause weakness, confusion, and rarely, abnormal heart rhythms to develop. You may be advised to have a blood test to check for these problems.
• Other problems such as: upset stomach, dizziness on standing (due to too low blood pressure), impotence (reversible on stopping treatment), and skin sensitivity to sunlight.

Other considerations

Most thiazide diuretics are taken once a day in the morning. Any reduction in blood pressure is maintained throughout the 24 hours by this once a day dose. But the effect of passing extra urine wears off within 12 hours. So, you will not have to get up in the night to make extra trips to the toilet. Indeed, the dose used to treat high blood pressure is quite low, and many people barely notice an increase in the amount of urine that they pass.

© EMIS and PIP 2004   Updated: July 2004   Review Date: August 2005   CHIQ Accredited