If you have high blood pressure and are not on a diuretic, or water pill, then you aren't doing everything you can to decrease your risk of having a stroke. That's the bottom-line message of a new study that shows people on other blood pressure drugs were more than twice as likely to have a stroke than people taking the diuretics known as thiazides.

"If I had high blood pressure, I would take a diuretic as a first-line treatment drug," says study author Bruce M. Psaty, MD, PhD. He is a professor of medicine and epidemiology at the University of Washington in Seattle.

The main reason for treating high blood pressure is to stave off complications such as a heart attack and stroke. Thiazide diuretics have been around for many years and, in fact, already are currently recommended as first-line treatment for high blood pressure because of how well they work.

"If you are a patient who has high blood pressure and you are not on a low-dose diuretic, it's a good idea to talk with you doctor and ask why not," Psaty says. For some patients, he says, there will be a good reason -- such as an allergy to the medication. But it is important that patients and doctors talk about the drug therapy that is best for them, he says.

Diuretics are very affordable, Psaty says. "An entire month's supply ... cost less than a Starbucks coffee," he says.

The most common thiazide is known by doctors as hydrochlorothiazide, or HCTZ. In addition to being taken alone, it is often found in a pill combined with other blood pressure medications. If you are taking a blood pressure medication that ends in -ide or -etic, then you are likely taking HCTZ.

For their study, Psaty and his fellow researchers looked at more than 3,000 people with high blood pressure. They found that people using some other blood pressure drug than a thiazide diuretic were 85% more likely to have a stroke caused by a blood clot than were those who were taking a thiazide diuretic.

They are not quite sure why thiazides seem to ward off a stroke better than other medications, but think it might have something to do with how diuretics attack high blood pressure. They say when the systolic blood pressure -- the top number -- is high, stroke is more likely than when the diastolic blood pressure -- the bottom number -- is high. Thiazide water pills are better at lowering the systolic blood pressure than other blood pressure medicines, and thus more effective at preventing stroke.

"This type of study is important," says David A. Olson, MD, staff neurologist at Georgia Regional Hospital in Atlanta and a medical adviser to WebMD. All medicines used to treat high blood pressure are not created equally, he says. The next step is to see if other investigators get the same results when they do similar studies.

"If these findings are replicated," Olson tells WebMD, "diuretics may be the treatment of choice for prevention of stroke."

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