Hydrochlorothiazide

Thiazide Diuretic Benzothiadiazine Diuretic

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 2-4 mg/kg q12h Duration: Variable; for edema, until resolution; for hypertension, chronic
Notes: For nephrogenic diabetes insipidus, dose may be 2-5 mg/kg q12h. For hypercalciuria, 2-4 mg/kg q12h.
Cat PO 2-4 mg/kg q12h Duration: Variable; for edema, until resolution; for hypertension, chronic
Notes: Use cautiously in cats with renal disease. For nephrogenic diabetes insipidus, dose may be 2-5 mg/kg q12h.
Horse PO 0.5-1 mg/kg q12h Duration: Variable; for edema, until resolution
Notes: May also be given IV at 0.5-1 mg/kg q12h. Monitor electrolytes.
Cattle PO 1-2 mg/kg q12h Duration: Variable; for edema, until resolution
Notes: Not commonly used; monitor for electrolyte imbalances.
Small Ruminants (Sheep/Goats) PO 1-2 mg/kg q12h Duration: Variable; for edema, until resolution
Notes: Not commonly used; monitor for electrolyte imbalances.
Rabbit PO 2-4 mg/kg q12h Duration: Variable; for edema, until resolution
Notes: Limited data; use with caution.

Clinical Indications & Species Uses

General Indications
  • Edema of various etiologies
  • Hypertension (adjunctive)
  • Nephrogenic diabetes insipidus
  • Hypercalciuria
Dog (Canine)
  • Edema associated with congestive heart failure
  • Hypertension (adjunctive therapy)
  • Nephrogenic diabetes insipidus
  • Hypercalciuria and calcium oxalate urolithiasis prevention
Cat (Feline)
  • Edema associated with congestive heart failure
  • Hypertension (adjunctive therapy)
  • Nephrogenic diabetes insipidus
  • Hypercalciuria and calcium oxalate urolithiasis prevention
Horse (Equine)
  • Edema (e.g., distal limb edema, ventral edema)
  • Hypertension (rarely used)
  • Nephrogenic diabetes insipidus (uncommon)
Cattle (Bovine)
  • Edema (e.g., udder edema, pulmonary edema)
  • Hypertension (rarely used)
Small Ruminants (Sheep / Goat)
  • Edema (e.g., udder edema, pulmonary edema)
  • Hypertension (rarely used)
Rabbit & Small Mammals
  • Edema (uncommon)
  • Hypertension (rarely used)

Pharmacology & Mechanism of Action

Drug Class: Thiazide Diuretic | Pharmacological Group: Benzothiadiazine Diuretic

Mechanism of Action: Hydrochlorothiazide inhibits the sodium-chloride symporter (NCC) in the distal convoluted tubule of the nephron, thereby reducing sodium reabsorption. This leads to increased excretion of sodium, chloride, and water, producing diuresis. It also increases potassium and bicarbonate excretion, and decreases urinary calcium excretion. The antihypertensive effect is initially due to plasma volume reduction, but with chronic use, it is attributed to reduced peripheral vascular resistance.

Pharmacodynamics: Hydrochlorothiazide produces diuresis within 1-2 hours after oral administration, with peak effect at 4-6 hours, and duration of action typically 6-12 hours. It reduces blood pressure by decreasing plasma volume and cardiac output initially, then by reducing peripheral vascular resistance. It also enhances the effect of other antihypertensive agents. In animals with nephrogenic diabetes insipidus, it reduces urine volume by promoting sodium and water reabsorption in the proximal tubule (paradoxical antidiuretic effect).

⚑ Pharmacokinetics Summary

Absorption: Hydrochlorothiazide is well absorbed from the gastrointestinal tract after oral administration. Bioavailability is approximately 60-70% in dogs and cats. Food may increase absorption.
Distribution: It is widely distributed in the body, crosses the placenta, and is distributed into milk. It does not significantly penetrate the blood-brain barrier. Volume of distribution is approximately 0.8 L/kg in dogs.
Metabolism: Hydrochlorothiazide is minimally metabolized in the liver; most of the drug is excreted unchanged in the urine.
Excretion: Excretion is primarily renal via glomerular filtration and active tubular secretion. In dogs, approximately 95% of the drug is excreted unchanged in urine. The elimination half-life is about 2-3 hours in dogs, but the diuretic effect may last longer.
Half-Life: Dogs: 2-3 hours; Cats: approximately 2.5 hours; Horses: 1.5-2 hours; Cattle: 2-3 hours.
Bioavailability: Oral bioavailability is approximately 60-70% in dogs and cats.
Protein Binding: Approximately 40-60% bound to plasma proteins in dogs and cats.

Available Formulations & Strengths

Oral Tablet 25 mg, 50 mg, 100 mg (PO)
Oral Solution 50 mg/mL (compounded) (PO)
Injectable Solution 25 mg/mL (as sodium salt, for IV use) (IV)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to hydrochlorothiazide or other sulfonamide-derived drugs
  • Anuria or severe renal impairment
  • Hepatic cirrhosis or severe hepatic disease (may precipitate hepatic encephalopathy)
  • Severe electrolyte imbalances (e.g., hyponatremia, hypokalemia)
  • Concurrent use with other potassium-depleting diuretics without careful monitoring
Warnings & Clinical Precautions:
  • Use with caution in patients with diabetes mellitus; may increase blood glucose.
  • Use with caution in patients with gout or hyperuricemia; may increase uric acid levels.
  • May cause electrolyte imbalances (hypokalemia, hyponatremia, hypochloremia, hypomagnesemia, hypercalcemia).
  • Monitor renal function and electrolytes periodically during therapy.
  • In animals with hepatic disease, may precipitate hepatic encephalopathy.
  • May cause photosensitivity in some species.
  • Use in pregnant animals only if clearly needed; crosses placenta.
  • In lactating animals, drug is excreted in milk; use with caution in nursing animals.
  • In food animals, observe withdrawal times.

Adverse Effects & Reactions

Common:

  • Polyuria
  • Polydipsia
  • Hypokalemia
  • Hyponatremia
  • Hypochloremia
  • Hypercalcemia
  • Hyperglycemia
  • Hyperuricemia
  • Gastrointestinal upset (vomiting, diarrhea, anorexia)

Serious / Severe:

  • Severe electrolyte imbalances
  • Cardiac arrhythmias (due to hypokalemia)
  • Acute pancreatitis (rare)
  • Hepatic encephalopathy in patients with liver disease
  • Renal dysfunction
  • Allergic reactions (skin rash, anaphylaxis)

Rare:

  • Blood dyscrasias (thrombocytopenia, leukopenia)
  • Photosensitivity
  • Interstitial nephritis
  • Pulmonary edema (noncardiogenic)

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Digoxin Thiazide-induced hypokalemia increases the risk of digoxin toxicity. High
Corticosteroids Additive potassium depletion. Moderate
NSAIDs Reduced diuretic and antihypertensive efficacy; increased risk of renal impairment. Moderate
ACE inhibitors Additive hypotensive effect; risk of hyperkalemia if potassium-sparing agents are used. Moderate
Lithium Reduced renal clearance of lithium, increasing risk of lithium toxicity. High
Sulfonylureas May reduce hypoglycemic effect; dose adjustment may be needed. Moderate
Amphotericin B Additive potassium depletion. Moderate

Overdose & Toxicity Management

Signs of Toxicity:

  • Excessive diuresis
  • Dehydration
  • Hypotension
  • Electrolyte imbalances (hypokalemia, hyponatremia, hypochloremia)
  • Lethargy
  • Weakness
  • Cardiac arrhythmias
  • Muscle cramps
  • Confusion (in severe cases)

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Discontinue the drug. Monitor vital signs and electrolytes. Replace fluids and electrolytes intravenously as needed. For severe hypokalemia, administer potassium chloride. For hypotension, use IV fluids and vasopressors if necessary. In cases of recent ingestion, consider gastric decontamination (emesis or activated charcoal) if appropriate. Provide cardiac monitoring and supportive care.

Food Animal Withdrawal Times

πŸ₯© Meat: 3 daysπŸ₯› Milk: 3 days

Withdrawal times are not established for hydrochlorothiazide in food animals. Use in food animals should be under veterinary supervision and follow label directions. The values provided are estimates based on similar diuretics; consult regulatory guidelines.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (20-25Β°C, excursions permitted to 15-30Β°C).

Handling & Special Conditions: Protect from moisture. Keep container tightly closed.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for human use; not FDA-approved for veterinary use, but may be used extra-label in animals.

Extra-Label (Off-Label) Use: In the US, extra-label use in food animals is permitted under AMDUCA with a valid veterinary-client-patient relationship, provided that withdrawal times are extended and residues do not exceed tolerance levels. However, hydrochlorothiazide is not approved for food animals, so extra-label use requires careful residue consideration.

Clinical Pearls & Practice Notes

πŸ’‘ Clinical Insights: Hydrochlorothiazide is a thiazide diuretic used in veterinary medicine primarily for the management of edema and hypertension. It is less potent than loop diuretics like furosemide but is useful for chronic therapy. In dogs and cats, it is commonly used as an adjunct to ACE inhibitors for heart failure and hypertension. It is also effective in nephrogenic diabetes insipidus, where it paradoxically reduces urine output. Monitor electrolytes, renal function, and blood pressure during therapy. Use with caution in patients with renal or hepatic disease. In food animals, observe withdrawal times and consider regulatory restrictions. Always follow label directions and veterinary guidance.

References & Literature

  • πŸ“š Plumb's Veterinary Drug Handbook
  • πŸ“š Papich Veterinary Pharmacology and Therapeutics
  • πŸ“š Compendium of Veterinary Products (CVP)