Chlortalidone, also known as chlorthalidone, is a thiazide-like diuretic drug used to treat high blood pressure, swelling including that due to heart failure, liver failure, and nephrotic syndrome, diabetes insipidus, and renal tubular acidosis. Because chlortalidone is reliably effective in most patients with high blood pressure, it is considered a preferred initial treatment. It is also used to prevent calcium-based kidney stones. It is taken by mouth. Effects generally begin within three hours and last for up to three days. Strong evidence suggests chlortalidone is more effective than hydrochlorothiazide for prevention of heart attack or stroke. Common adverse effects include low blood potassium, low blood sodium, high blood sugar, dizziness, and erectile dysfunction. Other adverse effects may include gout, low blood magnesium, high blood calcium, allergic reactions, and low blood pressure. Some reviews have found chlortalidone and hydrochlorothiazide to have a similar risk of adverse effects, while other reviews have found chlortalidone to have a higher risk. While it may be used in pregnancy it is a less preferred option. How it works is not completely clear but is believed to involve increasing the amount of sodium and water lost by the kidneys. Chlortalidone was patented in 1957 and came into medical use in 1960. It is on the World Health Organization's List of Essential Medicines. It is available as a generic medication. In 2020, it was the 133rd most commonly prescribed medication in the United States, with more than 4million prescriptions. Medical use High blood pressure Chlortalidone is considered a first-line medication for treatment high blood pressure. Some recommend chlortalidone over hydrochlorothiazide. A meta-analysis of randomized controlled trials found that chlortalidone is more effective than hydrochlorothiazide for lowering blood pressure, while the two drugs have similar toxicity. Trials of chlortalidone for high blood pressure found that lower doses of chlortalidone (e.g., 12.5 mg daily in ALLHAT study) had maximal blood pressure lowering effect and that higher doses did not lower it more. Chlortalidone and other thiazide diuretics are effective for lowering high blood pressure in persons with chronic kidney disease, although the risk of adverse effects is higher in these persons. Left ventricular hypertrophy Chlortalidone is used to treat left ventricular hypertrophy in the heart; it works chiefly by lowering blood pressure, and thereby reducing systemic vascular resistance. There is evidence that chlortalidone is superior to hydrochlorothiazide for reducing the mass of the left ventricle of the heart in