Hydrochlorothiazide: everything you need to know

Hydrochlorothiazide (brand names Microzide and Oretic) is a popular option to treat high blood pressure. It is a thiazide diuretic, a.k.a. a "water pill," that helps you get rid of excess salt and water.

Hydrochlorothiazide, often called HCTZ, acts on the kidney filtration system, which determines which salts get excreted in the urine and which stay in the body. HCTZ prevents the kidneys from reabsorbing sodium and chloride ions, allowing them to be removed in the urine. Water typically follows ions—the more ions that get excreted, the more water goes with them. Losing this fluid can help lower blood pressure and improve swelling.

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HCTZ is FDA-approved to treat the following conditions: 

• High blood pressure (hypertension)

• Swelling (edema)

High blood pressure

Almost 50% of all Americans have high blood pressure, also called hypertension (AHA, 2017). Since it doesn't usually cause symptoms, most people don't even realize that they have high blood pressure. For some people, treatment mainly entails lifestyle changes like a healthy diet, regular physical activity, and smoking cessation. However, others may need to start prescription blood pressure medications, also known as antihypertensive drugs.

So, why does it even need to be treated if there are no symptoms? Hypertension can lead to heart attacks, strokes, kidney disease, and other problems. HCTZ can help you get rid of excess water, thereby lowering your blood pressure and decreasing the workload on your heart. It can also be combined with other blood pressure medications, if appropriate. Talk to your healthcare provider about the best treatment options for you.

Swelling (edema)

Some medical conditions cause fluid retention and edema, which is swelling in the legs, ankles, feet, abdomen, or other places. Since HCTZ aids in the removal of excess fluid, it can help decrease swelling due to medical conditions like heart failure, liver disease, kidney disease, or taking corticosteroids for a long time (FDA, 2011).

Off-label uses

Off-label means that HCTZ was not specifically FDA-approved to treat a particular condition.

Some healthcare professionals may use it off-label to prevent kidney stones (MedlinePlus, 2019). When too much calcium gets into the urine, it can combine with other salts and minerals, and form stones. These stones are very painful when they pass through the urinary system. HCTZ can help because it prevents your kidneys from excreting calcium into the urine. Less calcium means hopefully fewer stones (UpToDate, n.d.).

HCTZ may also benefit people with diabetes insipidus, a medical condition marked by an imbalance of salts fluids in the body (UpToDate, n.d.). It is not the same as diabetes mellitus, which is characterized by high blood sugar. HCTZ affects how water and salts are excreted and reabsorbed, and may help break the cycle linked to diabetes insipidus of drinking and urinating too much (Bichet, 2019).

HCTZ is available both as a generic medication and under the brand names Microzide and Oretic. It is usually taken in pill form once daily and comes in 12.5 mg, 25 mg, and 50 mg strengths. You may also find HCTZ combined with other blood pressure medications (like amlodipine, lisinopril, and valsartan) in the same pill. Most prescription plans cover HCTZ, and it usually costs around $3–$6 for a 30-day supply (GoodRx.com).

Clinical trials looking at the frequency of adverse effects from HCTZ showed that most side effects are dose-related, and occurred more frequently at doses of 25 mg and above. People taking the 12.5 mg dose had side effects at about the same rate as those taking a placebo (DailyMed, 2014).

Common side effects include (DailyMed, 2014):

• Urinating more frequently

• Diarrhea or constipation

• Nausea/vomiting

• Loss of appetite

• Headache

• Weakness

• Vision problems

• Impotence

Serious side effects include (DailyMed, 2014):

• Low blood pressure (hypotension): HCTZ is often used to help lower blood pressure. Sometimes, your blood pressure can drop too low while taking this drug. Symptoms of low blood pressure include dizziness, fainting, blurred vision, fatigue, shallow breathing, rapid heart rate, and confusion. You may only notice your blood pressure drop after standing up from a sitting or laying down position—this is called orthostatic hypotension.

• Electrolyte imbalance: Hydrochlorothiazide may cause high blood sugar (hyperglycemia), low sodium (hyponatremia), low chloride (hypochloremic alkalosis), low potassium (hypokalemia), and low magnesium (hypomagnesemia) levels. Signs and symptoms of electrolyte imbalances include dry mouth, thirst, weakness, tiredness, muscle aches, irregular heartbeats, nausea, and vomiting. Low sodium levels can be life-threatening in some people.

• Jaundice: Rarely, people may experience liver problems and jaundice, which is yellowing of the skin and eyes.

• Kidney problems

• Severe allergic reactions: Some people may have a severe allergic reaction to HCTZ, causing hives, swelling of the tongue or throat, and shortness of breath. Severe skin rashes like Stevens-Johnson syndrome or toxic epidermal necrolysis may also occur.

• Angle-closure glaucoma

This list may not include all side effects and others may occur. Get medical advice from your healthcare provider or pharmacist for more information.

Talk to your healthcare provider about any other drugs that you are taking before starting HCTZ, including prescription drugs, over-the-counter medications, or supplements. Potential drug interactions include (DailyMed, 2014):

• Alcohol, barbiturates, narcotics: These drugs may interact with HCTZ and increase the risk of developing low blood pressure, especially orthostatic hypotension.

• Drugs to treat diabetes: If you are taking HCTZ along with insulin or oral medications to control your blood sugar, you may need to adjust your antidiabetic drug dose.

• Cholestyramine and colestipol: Both of these medications help lower cholesterol. However, they may decrease the effectiveness of HCTZ, and your dose of the diuretic may need to be adjusted.

• Corticosteroids and adrenocorticotropic hormone (ACTH) therapy: These therapies help treat adrenal insufficiency, a condition that can also cause high blood pressure. Taking HCTZ with these medications increases the risk of electrolyte imbalances, especially low levels of potassium; low potassium can lead to irregular heart rhythms. If you need to use these therapies together, you may need to take potassium supplements or eat more potassium-rich foods (like bananas and sweet potatoes).

• Lithium: Taking lithium (a mood stabilizer often used to treat bipolar disorder) with diuretics can be dangerous. HCTZ and other diuretics can decrease the amount of lithium removed by the kidneys—this increases the risk of lithium toxicity.

• Nonsteroidal anti-inflammatory drugs (NSAIDs): NSAIDs, like ibuprofen or naproxen, may lower HCTZ's effectiveness.

This list may not include all drug interactions with HCTZ, and others may exist. Get medical advice from your healthcare provider or pharmacist for more information.

HCTZ may increase the risk of side effects in certain groups of people. These groups should avoid using HCTZ or use it with caution and careful monitoring. Examples of these groups include (DailyMed, 2018):

• Pregnant women: Women often experience swelling while pregnant. However, if the swelling is due to a pathological condition, then treatment with HCTZ may be appropriate. HCTZ is considered pregnancy category B—this means that there are no proven risks in humans. Be aware that if it is taken during pregnancy, the baby may develop jaundice or one of the other adverse effects of hydrochlorothiazide.

• Women who are breastfeeding: HCTZ does enter breast milk. Nursing mothers should consult with their healthcare provider as the decision to take HCTZ involves weighing the risks to the baby with the benefits to the mother.

• People with gout: HCTZ may increase the level of uric acid (hyperuricemia) in the blood; this can lead to an episode of gout in susceptible people.

• People with liver disease: People with liver disease or poor liver function may experience neurological problems or coma on HCTZ.

• People with adrenal insufficiency: This group should not use HCTZ to lower blood pressure as this increases the risk of developing electrolyte imbalances, especially low potassium levels.

• People with diabetes: People with diabetes are often taking antidiabetic medications (like insulin) and monitoring their blood glucose levels. HCTZ may make it more difficult for them to control their blood glucose, and they may need to adjust their antidiabetic medication dose.

• People who have or who are at risk for angle-closure glaucoma: Angle-closure glaucoma usually only occurs in people with "narrow angles in the eye"—meaning that the front part of the eye is shallower than average. If you have this eye problem, HCTZ may trigger an episode of angle-closure glaucoma (high eye pressure, eye pain, eye redness, blurred vision, halos around lights) that can lead to permanent vision loss. You may need an eye exam before starting HCTZ.

• People with high calcium levels (hypercalcemia): If you have high blood calcium levels, you should avoid HCTZ because it decreases the amount of calcium excreted by the kidneys.

• People with kidney disease: This group is at higher risk for side effects with HCTZ. People whose kidneys do not produce urine (anuria) should not use this drug.

• People with sulfonamide ("sulfa") allergy: There is some evidence to suggest that allergy to one drug in the sulfonamide class may extend to other members. The data on this allergy cross-reactivity is not definitive, and people with a "sulfa" allergy should talk to their healthcare provider before taking HCTZ.

This list does not include all at-risk groups, and others may exist. Talk to your healthcare provider or pharmacist for more information.

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How we reviewed this article

Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

Current version

September 14, 2020

Written by

Chimene Richa, MD

Fact checked by

Mike Bohl, MD, MPH, ALM

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About the medical reviewer

Mike Bohl, MD, MPH, ALM

Dr. Mike is a licensed physician and a former Director, Medical Content & Education at Ro.