Hydrochlorothiazide is a diuretic (water pill) used primarily to treat high blood pressure and reduce fluid buildup in the body. It’s one of the most widely prescribed blood pressure medications in the United States and has been in use for decades. The drug works by helping your kidneys flush out extra sodium and water through your urine, which lowers blood pressure and relieves swelling.
High Blood Pressure
The most common reason doctors prescribe hydrochlorothiazide is to manage hypertension. The 2025 guidelines from the American Heart Association and American College of Cardiology list thiazide-type diuretics, including hydrochlorothiazide, as one of four first-line drug classes for treating high blood pressure in adults. The other three are calcium channel blockers, ACE inhibitors, and ARBs. All four have strong clinical trial evidence for lowering blood pressure, preventing cardiovascular disease, and being well tolerated.
Hydrochlorothiazide can be used on its own for mild to moderate hypertension or combined with other blood pressure drugs when a single medication isn’t enough. It’s often paired with an ACE inhibitor or ARB in a single combination pill, which simplifies treatment. The typical dose for blood pressure control is 25 to 50 mg per day.
Two other thiazide-type diuretics, chlorthalidone and indapamide, work in a similar way. A large clinical trial comparing hydrochlorothiazide to chlorthalidone found that switching from one to the other didn’t reduce rates of major cardiovascular events overall, though chlorthalidone may have a slight edge in people who already have cardiovascular disease. In practice, clinicians can choose among these agents based on the individual patient’s needs.
Fluid Retention (Edema)
Hydrochlorothiazide is also approved to treat edema, the medical term for excess fluid that causes swelling in your legs, ankles, feet, or abdomen. This fluid buildup happens in several conditions:
- Heart failure: When the heart can’t pump efficiently, fluid backs up in the body’s tissues.
- Liver cirrhosis: Scarring of the liver disrupts normal fluid balance, often causing abdominal swelling.
- Kidney disease: Conditions like nephrotic syndrome and chronic kidney failure impair the kidneys’ ability to filter fluid properly.
- Medication side effects: Corticosteroids and estrogen therapy can cause the body to retain fluid.
For edema, doses range from 25 to 100 mg daily, sometimes taken as a single dose and other times split into two. Depending on the situation, your doctor may have you take it every day or only on certain days of the week. The goal is to remove just enough fluid to relieve swelling without pushing the body too far in the other direction.
How It Works in Your Body
Hydrochlorothiazide targets a specific part of the kidney called the distal tubule. Normally, a transport channel in this section reclaims sodium and chloride from your urine and sends them back into the bloodstream. Hydrochlorothiazide blocks that channel. With more sodium staying in the urine, water follows it out, increasing urine volume.
Less fluid in the bloodstream means lower pressure against your artery walls, which is how it reduces blood pressure. It also means less fluid available to leak into tissues, which is how it reduces swelling. The effect is noticeable within a couple of hours of taking the pill and generally lasts 6 to 12 hours, which is why most people take it in the morning to avoid nighttime trips to the bathroom.
Common Side Effects
Because hydrochlorothiazide pushes sodium and water out through the kidneys, potassium tends to get swept out along with them. Low potassium is the most well-known side effect and can cause muscle cramps, weakness, or an irregular heartbeat if it drops significantly. Your doctor will likely check your potassium levels periodically and may recommend eating potassium-rich foods like bananas, oranges, and potatoes, or prescribe a potassium supplement.
Other common side effects include dizziness or lightheadedness (especially when standing up quickly), increased urination, and mild dehydration if you’re not drinking enough fluids. Some people also experience increased sensitivity to sunlight, so wearing sunscreen is a reasonable precaution.
Effects on Blood Sugar and Uric Acid
Hydrochlorothiazide can raise blood sugar levels, which matters most for people with type 2 diabetes. If you have diabetes and start this medication, you may need to monitor your blood sugar more carefully and adjust your diabetes treatment accordingly.
The drug can also increase uric acid levels in the blood. For most people this doesn’t cause symptoms, but if you have a history of gout or already run high uric acid levels, the extra bump could trigger a flare. This doesn’t mean you can’t take hydrochlorothiazide, but it’s something worth discussing with your doctor if gout has been a problem for you.
Skin Cancer Concerns
Earlier studies, mostly from Denmark, raised concerns about a possible link between long-term hydrochlorothiazide use and non-melanoma skin cancer, driven by the drug’s photosensitizing properties. However, more recent U.S. population-based research has painted a different picture. A study published in the AHA journal Hypertension compared skin cancer rates in hydrochlorothiazide users versus people taking ACE inhibitors across three large cohorts, including analyses that accounted for race and ethnicity. The researchers found no clear difference in skin cancer risk between the two groups. The concern hasn’t been fully dismissed, but the evidence is far less alarming than initial reports suggested.
Sulfa Allergy and Hydrochlorothiazide
Hydrochlorothiazide contains a sulfonamide chemical group, which sometimes raises concern for people who’ve had allergic reactions to sulfa antibiotics like sulfamethoxazole. The American Academy of Allergy, Asthma, and Immunology has addressed this directly: there is no clinically significant cross-reactivity between sulfonamide antibiotics and non-antibiotic sulfonamides like hydrochlorothiazide. This has been documented repeatedly over the past 20 years. A sulfa antibiotic allergy alone is generally not a reason to avoid hydrochlorothiazide.