Thiazide diuretics (water tablets) are used as a first line treatment to lower a high blood pressure or are combined with other classes of blood pressure treatment if the single drug alone does not control blood pressure. Thiazide diuretics include bendroflumethiazide, chlortalidone, cyclopenthiazide, indapamide, xipamide and metolazone.
How thiazide diuretics work
Thiazide diuretics work in two main ways:
- The main action of thiazide diuretics is to increase the loss of fluid and salts from the body via the kidneys. This is why they are popularly known as ‘water tablets’. This action lowers blood pressure by reducing the volume of fluid in the circulation.
- Thiazide diuretics also cause a mild dilation of small arteries (arterioles) in the circulation. This dilation lowers blood pressure by reducing the resistance caused as blood is pumped from an artery with wider diameter into those with a smaller bore, and provides more ‘space’ for fluid to expand into within the circulation.
Thiazides act within 1 or 2 hours of taking a tablet, and their action lasts for between 12 and 24 hours.
How effective are thiazide diuretics?
Relatively low doses of thiazide diuretics are used to lower blood pressure (higher doses are used to relieve swelling associated with chronic heart failure).
On average, thiazide diuretics reduce your systolic blood pressure (your upper reading) by 7.3 mmHg when used alone. Higher doses tend to have no further blood pressure lowering effect and are more likely to cause side effects.
Thiazide diuretic side effects
When you first start taking a thiazide diuretic, you will initially pass water more frequently than usual. This effect lasts for the first few days, then tends to disappear within a couple of weeks as dilation of the arterioles occurs.
The initial diuretic effect usually comes on quickly, within a few hours, so it’s a good idea to take your diuretic first thing in the morning to reduce trips to the bathroom at night.
Most people do not experience serious side effects.
Thiazides can cause mild gastro-intestinal disturbances, postural hypotension and, less commonly have been associated with gout, blood abnormalities, impotence, pancreatitis, liver problems, visual disturbances and skin reactions.
Photosensitivity reactions have been reported in people taking thiazide-like diuretics. If photosensitivity reactions (or other possible side effects) occur, tell your doctor.
If you have diabetes, it is important to monitor your blood glucose levels when taking a thiazide diuretic as they can affect your glucose control.
NB Some thiazide diuretics may give a positive reaction in doping tests used in athletics.
Patient information leaflets
To find out more about the most common thiazide diuretics, including their possible side effects, click on the following links. These will take you to a typical Patient Information Leaflet that is found inside a UK pack of these medicines.
Always read the patient information leaflet supplied with your own medicine as different preparations vary.
If your blood pressure is raised, self-monitoring is key to maintaining good control.
Click here for advice on choosing a blood pressure monitor to use at home.
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