Chlorthalidone is used in the management of hypertension. It is also indicated as an adjunctive treatment for edema associated with congestive heart failure, liver cirrhosis, and therapy involving corticosteroids or estrogens.

Chlorthalidone is a thiazide-like diuretic with antihypertensive properties, used in the treatment of high blood pressure. It may be administered alone or in combination with other antihypertensive agents. It is also employed as an adjunct therapy in edema related to renal disorders, mild to moderate congestive heart failure (NYHA Class II–III), cases where glomerular filtration rate exceeds 30 mL/min, ascites due to liver cirrhosis in stable patients, and edema associated with estrogen or corticosteroid therapy.

Chlorthalidone acts by inhibiting the reabsorption of sodium and chloride in the distal renal tubules, thereby promoting water excretion. The increase in urine output can lead to greater potassium loss. Although limited data are available regarding its absorption, its prolonged elimination half-life and clinical profile classify it as a long-acting thiazide-type diuretic. Longer-acting agents tend to produce more potassium depletion.

While it is considered a mild diuretic, its combination with loop diuretics can produce a stronger effect, as loop diuretics deliver a higher amount of sodium chloride to the distal tubules.

The initial blood pressure–lowering effect is due to reduced blood volume; however, sustained effects are related to decreased peripheral resistance through mechanisms that are not fully understood. High salt intake may counteract its antihypertensive effect.

Most of the absorbed chlorthalidone is excreted via the kidneys, with an average elimination half-life of about 50 hours. Only a small portion undergoes metabolism and biliary excretion. Within 120 hours, approximately 70% of the dose is eliminated through urine and feces, largely in unchanged form.

Therapy should be initiated with the lowest possible dose, and be titrated thereafter to gain maximum therapeutic benefit while keeping side effects to a minimum (e.g. determine the minimum effective maintenance dose for each patient). A single dose daily or every other day is given in the morning with food is recommended.

Hypertension: Usual adult dose is 25 to 50 mg daily. The clinically useful dosage range is 12.5 to 50 mg daily. Doses greater than 50 mg per day increase metabolic complications and are rarely of therapeutic benefit. For a given dose, the full effect is reached after 3 to 4 weeks. If the decrease in blood pressure obtained using doses of 25 or 50 mg/day proves inadequate, combined treatment with other antihypertensive drugs (such as beta-blockers and ACE inhibitors) is recommended. When adding an ACE inhibitor, Chlorthalidone is to be reduced or discontinued.

Edema of Specific Origin: The lowest effective dose is to be identified by titration. Maintenance doses should not exceed 50 mg/day and should be administered over limited periods only. The dosage should be individually adapted to the clinical picture and patient response. For long-term therapy, the lowest possible dosage sufficient to maintain an optimal effect should be employed; this applies particularly to elderly patients.

The therapeutic effect of Chlorthalidone occurs even without salt restriction and is well sustained during continued use.

The elderly: This is a suitable drug for treating hypertension in the elderly, in particular systolic hypertension. Dose of 50 mg daily, or less, should be used to avoid hypovolemia and hypokalemia.

Other interactions: In patients with impaired liver function or progressive liver disease, caution is required as even slight disturbances in fluid or electrolyte balance, or elevated ammonia levels, may trigger hepatic coma. Thiazide diuretics should be used carefully in patients who have undergone sympathectomy, as their antihypertensive effect may be enhanced. A cautious dosing approach is also recommended in individuals with severe coronary or cerebral arteriosclerosis.

Drug Interactions:

Antihypertensive Agents: Diuretics may enhance the effects of curare-like drugs and other antihypertensive medications (such as guanethidine, methyldopa, beta-blockers, vasodilators, and calcium channel blockers).

Digitalis: Thiazide-induced hypokalemia or hypomagnesemia can increase the risk of digitalis-related cardiac arrhythmias (see also precautions).

Corticosteroids: The potassium-lowering effect of diuretics may be intensified by corticosteroids, ACTH, and amphotericin.

Insulin and Oral Antidiabetic Agents: Dosage adjustments may be necessary due to altered glucose tolerance caused by chlorthalidone.

NSAIDs: Concurrent use of certain NSAIDs (e.g., indomethacin) may reduce the diuretic and antihypertensive effects of thiazides and may occasionally impair renal function in predisposed patients.

Curare Derivatives and Ganglionic Blocking Agents: Thiazides may increase sensitivity to these agents.

Allopurinol: Combined use may raise the risk of hypersensitivity reactions to allopurinol.

Amantadine: Co-administration may increase the likelihood of adverse effects associated with amantadine.

Antineoplastic Agents (e.g., cyclophosphamide, methotrexate): Thiazides may reduce renal excretion of these drugs and enhance their myelosuppressive effects. Anticholinergic agents (e.g., atropine, biperiden) may increase the bioavailability of thiazides by slowing gastrointestinal motility.

Cholestyramine: May reduce the absorption of thiazide diuretics, thereby decreasing their effectiveness.

Vitamin D: Combined use may increase serum calcium levels by reducing urinary calcium excretion.

Cyclosporine: Concurrent use may elevate the risk of hyperuricemia and gout-like symptoms.

Calcium Salts: May lead to hypercalcemia by increasing tubular calcium reabsorption.

Diazoxide: Thiazides may enhance the hyperglycemic effect of diazoxide.

Anuria, severe renal impairment (creatinine clearance below 30 mL/min), severe hepatic disease, persistent hypokalemia, conditions associated with excessive potassium loss, hyponatremia, hypercalcemia, symptomatic hyperuricemia (including a history of gout), and hypersensitivity to chlorthalidone or other sulfonamide-derived drugs or their components.

Use with caution in patients with renal disease or impaired liver function. If there is evidence of worsening renal function, such as rising BUN or serum creatinine levels, therapy should be discontinued. Chlorthalidone becomes ineffective as a diuretic when the glomerular filtration rate falls below 30 mL/min, and in such cases, loop diuretics may be more suitable.

Electrolytes: As with other thiazide diuretics, potassium loss (kaliuresis) with chlorthalidone is dose-dependent and may vary among individuals. At a daily dose of 25 mg, the average decrease in serum potassium is about 0.5 mmol/L. During long-term treatment, serum potassium should be monitored initially after one month and then periodically every 4–6 months if stable. Factors that may further reduce potassium levels include vomiting, diarrhea, malnutrition, renal impairment, liver cirrhosis, hyperaldosteronism, or concurrent use of corticosteroids or ACTH. Special attention is required in patients receiving digitalis or those with coronary heart disease, unless they are on ACE inhibitors or high-dose beta-agonists and maintain plasma potassium levels above 3.0 mmol/L.

Electrolytes and Metabolic Disorders: Common (especially at higher doses): hypokalemia, hyperuricemia, and increased blood lipids. Occasional: hyponatremia, hypomagnesemia, and hyperglycemia. Rare: hypercalcemia, glycosuria, worsening of diabetic metabolic state, and gout.

Isolated cases: Hypochloremic alkalosis.

Dermatology: Occasional: urticaria and other skin rashes. Rare: photosensitivity.

Liver: Rare: intrahepatic cholestasis or jaundice.

Cardiovascular: Occasional: postural hypotension, which may be aggravated by alcohol, anesthetics, or sedatives. Rare: cardiac arrhythmias.

CNS: Occasional: dizziness, slowed thinking, and reduced reaction time.

Pregnancy: Chlorthalidone, like other diuretics, may reduce placental blood flow. It does not prevent or alter the course of preeclampsia and should not be used for treating hypertension during pregnancy. Use in other conditions (e.g., heart disease) should generally be avoided, especially in the first trimester, unless the benefits outweigh the risks.

Lactation: Chlorthalidone is excreted into breast milk at approximately 4% of maternal plasma levels. Its use in breastfeeding mothers is therefore not recommended.

Renal impairment: Dose adjustment is required in moderate renal impairment (e.g., dosing every 24–48 hours), and it should be avoided in severe renal failure.

Liver disease: There is a risk of hepatic encephalopathy in patients with liver cirrhosis and ascites.

Use in pregnancy: It is preferable to avoid chlorthalidone as it crosses the placenta.

Use in lactation: Significant amounts may pass into breast milk and may suppress lactation; therefore, it should not be used in breastfeeding mothers.

Acute overdose may present with nausea, weakness, dizziness, and disturbances in electrolyte balance. The oral LD50 in animal studies is greater than 25,000 mg/kg. There is no specific antidote; gastric lavage followed by supportive care is recommended. Intravenous fluids with electrolytes may be required in some cases.

Thiazide diuretics & related drugs

Store below 30°C, protected from light and moisture. Keep out of reach of children.