Thiazide diuretics are a class of water pills that lower blood pressure by helping your kidneys flush out extra sodium and water. They’re one of four first-line treatments for high blood pressure recommended in the 2025 American Heart Association/American College of Cardiology guidelines, and they’ve been a cornerstone of blood pressure management for decades. In most people, thiazides reduce systolic blood pressure (the top number) by about 8 to 10 mmHg.
How Thiazides Work in Your Kidneys
Your kidneys filter blood through millions of tiny structures called nephrons. As fluid passes through each nephron, the kidney reabsorbs most of the sodium and water back into the bloodstream. Thiazide diuretics block a specific sodium transporter in a part of the nephron called the distal convoluted tubule. When sodium can’t be reabsorbed there, it stays in the urine, pulling water along with it. The result is more urine output and less fluid in your bloodstream, which lowers the pressure on your artery walls.
This is different from loop diuretics (like furosemide), which act on a different, earlier part of the nephron and pull out much more fluid. Loop diuretics are generally reserved for conditions that cause significant fluid buildup, like heart failure. Thiazides produce a gentler, more sustained effect that makes them better suited for everyday blood pressure control.
Common Thiazide Medications
The most widely prescribed thiazide and thiazide-like diuretics include:
- Hydrochlorothiazide (brand names Microzide, Inzirqo), the most commonly used option
- Chlorthalidone (Thalitone), which some guidelines consider the preferred choice for blood pressure management due to its longer duration of action
- Indapamide, a thiazide-like diuretic often grouped with traditional thiazides
- Chlorothiazide (Diuril)
- Metolazone, sometimes used in combination with other diuretics for resistant fluid retention
The term “thiazide-type diuretic” in medical guidelines collectively refers to hydrochlorothiazide, chlorthalidone, and indapamide, since these three have the strongest evidence behind them.
What Thiazides Are Prescribed For
High blood pressure is by far the most common reason you’d be prescribed a thiazide. They’re used alone or combined with other blood pressure medications. But thiazides also treat fluid retention (edema) caused by heart, kidney, or liver disease, as well as swelling triggered by certain medications like corticosteroids or estrogen therapy.
Thiazides have a couple of lesser-known uses, too. They can help prevent calcium-based kidney stones in people with high calcium levels in their blood, because they reduce the amount of calcium excreted in urine. They’re also sometimes prescribed for diabetes insipidus, a rare condition where the kidneys produce excessive amounts of dilute urine.
Side Effects and Electrolyte Changes
Because thiazides change how your kidneys handle minerals, electrolyte imbalances are the most important side effects to understand. The three key shifts are:
Low potassium. By pushing more sodium into the urine, thiazides also increase potassium loss. Low potassium can cause muscle cramps, weakness, and irregular heartbeats. Your doctor may recommend potassium-rich foods, a potassium supplement, or a combination pill that includes a potassium-sparing diuretic to offset this.
Low sodium. In one study of 951 outpatients taking thiazides, 14% developed low sodium levels. Symptoms can include headaches, confusion, nausea, and fatigue. Older adults and people who drink large amounts of water are at higher risk.
Elevated calcium. Unlike loop diuretics, which waste calcium, thiazides cause the kidneys to hold onto it. This is actually helpful for preventing kidney stones, but in some people it can push calcium levels too high.
Other metabolic effects matter as well. Thiazides can raise blood sugar, which is particularly relevant if you have diabetes or prediabetes. Insulin or oral diabetes medication doses may need adjusting. They also raise uric acid levels by concentrating the remaining body fluid and making it harder for the kidneys to excrete uric acid. This can trigger gout, a painful form of arthritis caused by uric acid crystal deposits in the joints.
Less commonly, some people experience dizziness from low blood pressure (especially when standing up quickly), increased sensitivity to sunlight, or mild stomach upset.
Who Should Avoid Thiazides
Thiazides are not appropriate for everyone. People who produce no urine (a condition called anuria) cannot take them. Because thiazides are chemically related to sulfonamide drugs, anyone with a known hypersensitivity to sulfonamide-derived medications should avoid them.
Several conditions call for extra caution. If you have significant kidney disease, thiazides may not work well and can worsen kidney function. People with liver disease need careful monitoring, because even small shifts in fluid and electrolyte balance can trigger serious complications. There have also been reports of thiazides worsening or activating lupus.
One rare but notable side effect: thiazides can occasionally cause sudden blurred vision or eye pain from a type of acute glaucoma. This typically happens within hours to weeks of starting the medication and requires prompt medical attention.
What Monitoring Looks Like
If you start a thiazide, expect routine blood work. Current guidelines recommend checking potassium, sodium, and kidney function (creatinine) at least once a year. Many doctors will check these more frequently when you first start the medication or after a dose change, typically within a few weeks. If you have diabetes, your blood sugar levels will also need closer attention, since thiazides can push them upward.
You’ll likely notice increased urination, especially in the first few days. Most people find this settles into a predictable pattern. Taking your dose in the morning helps avoid nighttime trips to the bathroom.
How Thiazides Fit Into Blood Pressure Treatment
The 2025 AHA/ACC guidelines place thiazide-type diuretics alongside calcium channel blockers, ACE inhibitors, and ARBs as the four classes recommended for initial blood pressure treatment. All four have strong evidence for lowering blood pressure, preventing cardiovascular disease, and being well tolerated. The choice between them often depends on your other health conditions, side effect profile, and how your body responds.
Among thiazides specifically, chlorthalidone is often highlighted as a preferred option over hydrochlorothiazide. Chlorthalidone lasts longer in the body, providing more consistent blood pressure control over a full 24-hour period. That said, hydrochlorothiazide remains the most widely prescribed thiazide in practice, partly because of its long track record and availability in many combination pills.
For people whose blood pressure doesn’t respond to a single medication, thiazides are frequently added as a second or third agent. They complement other blood pressure drugs well, and many combination pills pair a thiazide with an ACE inhibitor or ARB in a single tablet to simplify your routine.