This information is for people who have high blood pressure. It tells you about a group of drugs called diuretics, a treatment for high blood pressure. It is based on the best and most up-to-date research.
Do they work?
Yes. If you have high blood pressure (also called hypertension), drugs called diuretics can lower your blood pressure and help keep it down.
There is also a strong chance that taking a diuretic will reduce your risk of having a heart attack or a stroke or getting heart failure.
Diuretics are as good at lowering blood pressure as other drugs called beta-blockers, ACE inhibitors, calcium channel blockers, alpha-blockers, and angiotensin II receptor blockers.
What are they?
Diuretics are drugs used to lower blood pressure. They are sometimes called water tablets because they work by flushing excess salt and water from your body in your urine.
A group of diuretics called thiazides are usually used to treat high blood pressure. Some of the most common ones (and their brand names) are:
You may need to take more than one medicine to bring your blood pressure down. Most people need at least two drugs. 
Diuretics are often used with ACE inhibitors and beta-blockers because they can make these drugs work better. 
How can they help?
Diuretics can lower your blood pressure and keep it down.
Studies looking at thiazide diuretics showed that they can reduce the top number of your blood pressure reading (systolic pressure) by around 12 to 16 points and the bottom number (diastolic pressure) by 5 to 10 points.  To find out more about the numbers, see Understanding your blood pressure reading.
Reducing your blood pressure reduces your risk of having a heart attack, a stroke, or heart failure. It also reduces your risk of dying from one of these conditions.   
Diuretics work just as well as other drugs that lower blood pressure such as beta-blockers, ACE inhibitors, calcium channel blockers, alpha-blockers, and angiotensin II receptor blockers.      
Diuretics may work better than some other drugs at preventing heart failure, although they may not be as good as calcium channel blockers at cutting your risk of a stroke.  
How do they work?
You may have heard diuretics called water tablets. That's because they flush out excess salt and water from your body, in your urine. This takes some of the water out of your blood. As the amount of fluid in your blood vessels falls, so does your blood pressure.
Diuretics also open up the arteries (the blood vessels that carry blood from the heart around the body), making it easier for blood to flow through under less pressure.