Hydrochlorothiazide is a diuretic, or “water pill,” a drug that helps the body eliminate excess water and sodium in the form of urine. An older drug in the class known as thiazide diuretics, it is available under six brand names and in multiple combinations with an angiotensin II receptor blocker (ARB) or angiotensin-converting enzyme (ACE) inhibitor, which are two other classes of blood pressure medications.
What Is Hydrochlorothiazide Used For?
Hydrochlorothiazide is among the most common first-line medications to treat high blood pressure (hypertension). It is also used to reduce swelling (edema).
“The vast majority of time we use it for hypertension, particularly in older individuals, who tend to have a better response to diuretics, compared with younger people,” says Cleveland Clinic preventive cardiologist Luke J. Laffin, MD. “They particularly like the convenience of the fixed-dose combinations because it’s one less pill they have to take.”
How Hydrochlorothiazide Works
Sodium attracts and holds on to water. Hydrochlorothiazide works by preventing the kidneys from resorbing sodium and chloride back into the bloodstream. Instead, excess sodium is peed out, and water follows. The increase in urine output is called diuresis, and it lowers the volume of blood in the body. With less blood to pump, the heart does not have to work as hard, and blood pressure drops.
Hydrochlorothiazide lowers blood pressure in another way, as well. “For decades, we have known that hydrochlorothiazide, when taken for a prolonged period of time, also leads to vasodilation. It contributes to blood pressure lowering by relaxing the blood vessels,” Dr. Laffin says.
The drug does not work for everyone, so physicians have alternatives on hand. “When used strictly for blood pressure lowering, hydrochlorothiazide is not the most effective diuretic. Chlorthalidone or indapamide can be better,” he says. “The reason hydrochlorothiazide is ubiquitous is that it is included in many combination medications for hypertension.”
When hydrochlorothiazide is effective, blood pressure should drop within a week or two.
How to Take Hydrochlorothiazide
Hydrochlorothiazide comes in the form of a pill or tablet that can be taken with or without food. Because it increases urination, you may want to take it in the morning, so your sleep is not interrupted by frequent trips to the bathroom.
If you miss a dose, take it as soon as you remember. If it’s almost time for your next dose, skip the missed dose and just take that dose. Ask your doctor how much liquid you should consume every day while taking this diuretic, so that you can eliminate excess water without becoming dehydrated.
Hydrochlorothiazide Side Effects
Because hypertension is most often a symptomless disease, hydrochlorothiazide won’t make you feel any better. However, it should not make you feel any worse. Fortunately, hydrochlorothiazide side effects are not common.
Hydrochlorothiazide can interact with many medications. Because hydrochlorothiazide has been widely used for decades, most pharmacists and physicians have potential interactions on their radar. “Be sure your doctors know all the prescription medications you take, as well as over-the-counter drugs, dietary supplements, vitamins and herbal medications,” Dr. Laffin says.
In some individuals, hydrochlorothiazide may cause an allergic reaction, kidney injury, low blood pressure, sudden eye pain or blurred vision. If you experience any of these reactions, stop taking the medication and call your doctor immediately.
Hydrochlorothiazide also can cause dehydration—a sign that it is working too well. “Maintaining a balance between the amount of fluid coming in and going out is sometimes a matter of trial and error. If you feel faint or lightheaded, are unusually thirsty, have a dry mouth or find that your urine is dark, drink water and call your doctor,” Dr. Laffin advises. “Your dose may need to be adjusted, or you may need to increase your fluid intake.”
Hydrochlorothiazide can also cause potassium levels to drop. Symptoms of low potassium include muscle pain or cramps, unusual weakness or fatigue, constipation and a fast or irregular heartbeat.
Hydrochlorothiazide may raise blood sugar levels a little bit, which may be a concern to people with diabetes. It also may increase uric acid levels, which can lead to gout. “For these reasons we monitor patients by checking their blood within a couple weeks of starting hydrochlorothiazide,” Dr. Laffin says.
What Are the Long-Term Side Effects of Hydrochlorothiazide?
Long-term side effects of hydrochlorothiazide are rare—a good thing, since you will probably take the drug for life or, as Dr. Laffin explains, “until the risks outweigh the benefits.”
The primary long-term concern of the drug is that it can increase your skin’s sensitivity to the sun,” he advises. “If you can’t or won’t avoid the sun, be sure to apply sunscreen every two hours and wear a hat and protective clothing.”
