Hydrochlorothiazide (HCTZ) does not cause kidney stones. It’s actually one of the most commonly prescribed medications to prevent them, particularly calcium-based stones. However, the picture is more complicated than it appears. HCTZ can trigger metabolic side effects, including elevated uric acid and low potassium, that may work against its stone-preventing benefits in some people.
How HCTZ Reduces Kidney Stone Risk
HCTZ is a thiazide diuretic, most often prescribed for high blood pressure. But it has a second use: it directly reduces the amount of calcium your kidneys dump into your urine. Since about 80% of kidney stones are made primarily of calcium, less calcium in the urine means fewer raw materials for stones to form.
This isn’t just a side effect of producing more urine. HCTZ acts on a specific part of the kidney’s filtering system to increase calcium reabsorption back into the bloodstream, so less ends up in your urine. Research confirms this is a persistent, direct tubular effect rather than something mediated by vitamin D or parathyroid hormone changes.
The Evidence Is Weaker Than Expected
For decades, thiazide diuretics were considered a cornerstone of kidney stone prevention. Older pooled analyses of clinical trials found that thiazides cut the relative risk of stone recurrence by roughly 50%, with a 21% absolute risk reduction. A Cochrane review of patients with high urinary calcium found those on thiazides were significantly more likely to stay stone-free.
Then came the NOSTONE trial, published in the New England Journal of Medicine in 2023, which challenged that confidence. This large, well-designed study randomized 416 recurrent stone formers to receive either HCTZ (at 12.5 mg, 25 mg, or 50 mg daily) or a placebo, with a median follow-up of about three years. The results were surprising: stone recurrence rates were 59% in the placebo group, 59% at 12.5 mg, 56% at 25 mg, and 49% at 50 mg. None of these differences reached statistical significance. The researchers concluded that HCTZ did not substantially reduce stone recurrence at any of the tested doses.
That said, this finding hasn’t settled the debate. A closer analysis of the NOSTONE data looking specifically at stones visible on imaging (rather than just symptomatic episodes) did show a meaningful, roughly 50% reduction in radiologic recurrence at higher doses. This is consistent with how HCTZ has traditionally been used. The disconnect may come down to the fact that preventing new stone formation on a scan and preventing a painful stone episode aren’t exactly the same thing.
How HCTZ Could Work Against You
Here’s where the “can HCTZ cause kidney stones” question gets interesting. While HCTZ lowers urinary calcium, it also triggers metabolic shifts that can promote stone formation through other pathways.
Low potassium and reduced citrate: HCTZ commonly causes potassium loss. Low potassium, whether it develops on its own or from HCTZ, is a risk factor for low urinary citrate levels. Citrate is one of your body’s natural stone inhibitors; it binds to calcium in the urine and prevents crystals from forming. If HCTZ drives your potassium down and your citrate drops along with it, the benefit of lower urinary calcium could be partially or fully canceled out. Some experts believe this effect may explain why the NOSTONE trial showed weaker results than older studies, many of which routinely paired thiazides with potassium supplements.
Elevated uric acid: HCTZ raises blood uric acid levels, which can increase the risk of gout and, at least theoretically, uric acid kidney stones. Uric acid stones make up about 5 to 10% of all kidney stones and form when urine is too acidic or contains too much uric acid. If you already have a tendency toward uric acid stones, this side effect deserves attention.
Other concerns with long-term use: In the NOSTONE trial, patients on HCTZ had higher rates of newly diagnosed diabetes, low potassium, gout, and worsened kidney function compared to placebo. Because stone prevention typically means years of treatment, often starting in young adulthood, these risks accumulate. Current expert opinion increasingly favors a restrictive approach: trying HCTZ for two to three years and reassessing whether it’s actually changing stone activity before committing to indefinite use.
Chlorthalidone May Work Better
Not all thiazide diuretics are equal for stone prevention. Chlorthalidone, a longer-acting relative of HCTZ, appears to lower urinary calcium more effectively at the same dose. In a head-to-head comparison, patients on 25 mg of chlorthalidone saw a 41% reduction in 24-hour urinary calcium, while those on 25 mg of HCTZ saw only a 21% reduction. At the lowest dose of 12.5 mg, neither drug significantly lowered urinary calcium. Most of the older trials that showed strong stone prevention benefits used chlorthalidone or indapamide rather than HCTZ, which adds another layer of explanation for NOSTONE’s lukewarm results.
Why Diet Matters Alongside HCTZ
If you’re taking HCTZ for stone prevention, your salt intake directly affects how well it works. Sodium and calcium share transport pathways in the kidney. When you eat a lot of salt, your kidneys excrete more sodium and pull more calcium along with it, increasing urinary calcium levels and partially undoing what HCTZ is trying to accomplish. Research on patients combining HCTZ with dietary sodium restriction found significant decreases in urinary calcium excretion, reinforcing that the drug works best when paired with lower salt intake.
This is one of the most practical takeaways for anyone on HCTZ for stones: keeping sodium intake moderate (generally under 2,300 mg per day) amplifies the drug’s calcium-lowering effect. Without that dietary piece, HCTZ may not move the needle enough to matter.
The Bottom Line on HCTZ and Stones
HCTZ does not cause calcium kidney stones. Its primary pharmacologic effect is the opposite: it lowers urinary calcium. But its secondary metabolic effects, particularly potassium depletion leading to lower citrate and increased uric acid, can create conditions that favor stone formation through different mechanisms. The net effect depends on the dose, whether potassium levels are maintained, what type of stones you form, and how much sodium is in your diet. Recent evidence suggests the benefit of HCTZ alone, without potassium supplementation and dietary changes, is smaller than previously thought, and chlorthalidone may be the stronger option within this drug class.