The lowest FDA-approved starting dose of chlorthalidone is 12.5 mg once daily. Some clinicians prescribe an even lower dose of 6.25 mg by splitting a tablet in half, though this is considered off-label. Both doses are used in clinical practice, and understanding the differences between them can help you make sense of what your prescription means and why your dose may be set where it is.
FDA-Approved Dose Range
The FDA lists 12.5 mg as the lowest recommended starting dose for chlorthalidone, taken once daily with food. From there, the dose can be doubled every two to four weeks based on how well blood pressure responds, up to a maximum of 100 mg per day. The 2025 ACC/AHA blood pressure guidelines list a usual dose range of 12.5 to 25 mg daily for most adults with high blood pressure.
Generic chlorthalidone tablets are commercially available in 25 mg and 50 mg strengths. You’ll notice there’s no manufactured 12.5 mg tablet. To get a 12.5 mg dose, a 25 mg tablet is split in half. These tablets can be safely split, and many people do this routinely either with their fingers or a pill cutter.
The Off-Label 6.25 mg Dose
Although 12.5 mg is the official floor, a growing body of clinical evidence supports using 6.25 mg, which is a quarter of a standard 25 mg tablet. This ultra-low dose has been studied in both single-drug and combination therapy and has shown meaningful blood pressure reduction. Research published in The Lancet Global Health found that 6.25 mg of chlorthalidone was actually superior to 12.5 mg of hydrochlorothiazide (the most commonly prescribed thiazide diuretic) at controlling blood pressure.
This makes more sense when you consider that chlorthalidone is significantly more potent than hydrochlorothiazide. A meta-analysis cited by the American Heart Association found the two drugs have roughly a 3:1 potency ratio, meaning 25 mg of chlorthalidone does the work of about 75 mg of hydrochlorothiazide. Since 12.5 mg of hydrochlorothiazide is one of the most frequently prescribed diuretic doses in combination pills, the therapeutic equivalent in chlorthalidone is just 6.25 mg.
Why Lower Doses Matter
The main reason to care about dose is side effects, particularly potassium loss. Chlorthalidone causes your kidneys to excrete more potassium along with the excess sodium and water it flushes out. This effect is dose-dependent: the higher the dose, the greater the risk. A large observational study published in JAMA Internal Medicine found that even at the low 12.5 mg dose, chlorthalidone carried a notably higher rate of low potassium compared to 25 mg of hydrochlorothiazide. In one of the databases analyzed, about 6.3% of chlorthalidone users developed low potassium versus 1.9% of hydrochlorothiazide users.
Low potassium can cause muscle cramps, fatigue, and in more serious cases, heart rhythm problems. Starting at the lowest effective dose minimizes this risk while still delivering blood pressure reduction. If you’re already on chlorthalidone and experiencing muscle cramps or unusual fatigue, the dose may be part of the conversation with your prescriber.
How Chlorthalidone Compares to Other Diuretics
Chlorthalidone is often grouped with hydrochlorothiazide as a “thiazide-type” diuretic, but the two aren’t interchangeable milligram for milligram. Chlorthalidone lasts longer in the body and is roughly three times as potent. This is why chlorthalidone doses look so much smaller on paper. If you’ve been switched from hydrochlorothiazide 25 mg to chlorthalidone, a dose of 12.5 mg or even lower would be a reasonable equivalent, not a 25 mg tablet.
The American Heart Association has stated that chlorthalidone should be the preferred thiazide-type diuretic for hypertension, citing its longer duration of action and stronger blood pressure reduction, especially when paired with other blood pressure medications. Its potency advantage is also why such small doses remain clinically useful.
Getting a Low Dose in Practice
If your prescription is for 12.5 mg, you’ll typically receive 25 mg tablets with instructions to split them. A pill cutter gives you the cleanest break, especially since chlorthalidone tablets are on the smaller side. For a 6.25 mg dose, you’d quarter a 25 mg tablet, which is less precise but workable for a medication with a wide therapeutic window and long duration of action.
The brand-name product Hemiclor is available in a 12.5 mg tablet strength, which eliminates the need for splitting if your insurance covers it. For most people, though, generic 25 mg tablets split in half remain the most common and affordable way to take the lowest standard dose.