Prescription diuretics, the oldest—and some of the cheapest—of all high blood pressure drugs, are also one of the most widely used types of medications in the U.S. In fact, doctors in the U.S. write about 49 million prescriptions per year for one—hydrochlorothiazide—alone.

Commonly known as water pills, all diuretics have essentially the same result, removing sodium and extra fluid from the body.

They are a mainstay of treatment for high blood pressure and are recommended (often in combination with other medication) by the American Heart Association and the American College of Cardiology for those with high blood pressure that’s not well-controlled by lifestyle changes alone and for people who are retaining excess fluid.

Is it safe (and effective) to take prescription diuretics on a daily, long-term basis for high blood pressure, as millions of people do? For most people, yes, as long as they take the right steps. Here’s the information that will help you.

Different Types, Different Uses

There are three types of diuretics, and some are available in combination with others.

The so-called thiazides, such as hydrochlorothiazide (Hydrodiuril, Microzide, and generic) and chlorthalidone (generic only), are the diuretics most commonly prescribed for high blood pressure.  

Loop diuretics such as bumetanide (generic only), furosemide (Lasix and generic), and torsemide (Demadex and generic) are often prescribed for heart failure. This occurs when the heart muscle becomes too weak to pump sufficient blood to meet the body’s needs for blood and oxygen.

The third type, potassium-sparing diuretics such as amiloride (Midamor and generic) and triamterene (Dyrenium) are generally used with other diuretics for the prevention or treatment of low potassium levels, or hypokalemia.

Do Diuretics Work Well for High Blood Pressure?

These medications, which have been in use for more than half a century, are widely considered to be effective for lowering high blood pressure.

In one major, multi-center study of 33,357 people with high blood pressure, 68 percent of those who used the thiazide diuretic chlorthalidone brought their blood pressure levels down to a healthy range (below 140/90). They also reduced their risk for cardiovascular events—such as heart attacks and strokes—by 12 percent and the risk for heart failure by 21 percent. 

However, there are unanswered questions about whether one thiazide diuretic may be better than another.  

Currently, hydrochlorothiazide is by far the most common kind of thiazide diuretic prescribed for high blood pressure. But some research suggests that chlorthalidone may be more effective at preventing heart attack and stroke.

Until recently, there have been no head-to-head studies on the two medications. Last summer, the Diuretic Comparison Project (DCP), sponsored by the Veterans Administration, began comparing the two medications in people with high blood pressure. The researchers hope to enroll up to 13,500 study subjects.

This doesn't mean that you should switch from one thiazide to another, however. We may not see results from DCP for four years, so for now, experts say, take your prescribed medication as directed.

Sussing Out Safety

For the most part, diuretics are safe for long-term use, according to Consumer Reports' chief medical adviser Marvin M. Lipman, M.D. But like all medications, they can have side effects.

Most diuretics (with the exception of the potassium-sparing type) can reduce levels of the mineral potassium in your bloodstream, which can cause fatigue, muscle cramps, and weakness. (Normal potassium levels are 3.6 to 5.2 millimoles per liter of blood, or mmol/L).

To guard against potassium problems, your doctor will monitor levels as part of annual (or semi-annual) blood tests, notes cardiologist Vincent Bufalino, M.D., a spokesman for the American Heart Association, president of the Chicago-area Advocate Medical Group, and a clinical assistant professor of medicine at Loyola University-Stritch School of Medicine.

If your potassium levels fall too much, your doctor may recommend a potassium supplement or switch you to a potassium-sparing diuretic.

(Whether or not you take a daily diuretic, and as long as your kidneys are functioning normally, it’s wise to include potassium-rich foods such as baked sweet or white potatoes, spinach, white beans, winter squash, cantaloupe, bananas, milk, and yogurt in your daily diet—most of us get just half of the 4,700 mg per day recommended by the Institute of Medicine.)

And though it's a rare occurrence, call your doctor immediately or get emergency treatment if you are taking a diuretic and experience cramps or vomiting, or your pulse or heart rate increases dramatically—higher than about 100 beats per minute. This may indicate dangerously low potassium levels.

Diuretics can also boost the likelihood of a form of arthritis known as gout, so alert your healthcare provider if you notice a sudden painful inflammation of your big toe or knee.

In addition, these drugs may increase your risk for type 2 diabetes or make controlling your blood sugar more challenging. This may be, in part, because lower-than-normal potassium levels interfere with the body’s ability to produce insulin, a hormone we need to regulate blood sugar.  

But there may be lifestyle factors at work as well: “For the most part, increases in blood sugar go along with weight gain and inactivity—it’s usually not just the diuretic,” says Bufalino.