Losartan Potassium — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	0.5-1.5 mg/kg	q24h	Duration: Chronic (months to years) Notes: Start at 0.5 mg/kg q24h; titrate up to 1.5 mg/kg based on blood pressure and renal parameters. Monitor renal function and electrolytes.
Cat	PO	0.5-1.0 mg/kg	q24h	Duration: Chronic Notes: Commonly used dose is 0.5 mg/kg q24h; may increase to 1 mg/kg if needed. Use with caution in cats with renal insufficiency.
Horse	PO	0.5-1.0 mg/kg	q12-24h	Duration: Variable Notes: Limited data; experimental use only. Monitor blood pressure and renal function.
Rabbit	PO	1-2 mg/kg	q24h	Duration: Variable Notes: Extrapolated from other species; use with caution.

Clinical Indications & Species Uses

General Indications

Management of hypertension
Reduction of proteinuria in chronic kidney disease
Adjunctive therapy in heart failure

Dog (Canine)

Hypertension (systemic)
Chronic kidney disease with proteinuria
Heart failure (adjunctive therapy)
Glomerular disease

Cat (Feline)

Hypertension (systemic)
Chronic kidney disease with proteinuria
Hyperthyroidism-associated hypertension (off-label)

Rabbit & Small Mammals

Hypertension (experimental)
Chronic kidney disease (experimental)

Pharmacology & Mechanism of Action

Drug Class: Angiotensin II Receptor Blocker (ARB) | Pharmacological Group: Renin-Angiotensin-Aldosterone System (RAAS) Inhibitor

Mechanism of Action: Losartan is a selective, competitive antagonist of the angiotensin II type 1 (AT1) receptor. By blocking the binding of angiotensin II to AT1 receptors, it inhibits the vasoconstrictor and aldosterone-secreting effects of angiotensin II, leading to vasodilation, reduced aldosterone release, and decreased sodium and water retention. This results in lowered blood pressure and reduced cardiac afterload. Losartan also has a uricosuric effect by inhibiting the urate transporter in the renal proximal tubule, which may be beneficial in some species.

Pharmacodynamics: Losartan produces dose-dependent reductions in blood pressure in hypertensive animals. It also reduces proteinuria in chronic kidney disease by decreasing intraglomerular pressure and improving renal hemodynamics. In heart failure, it reduces preload and afterload, improving cardiac output. The antihypertensive effect is maximal at 3-6 hours after oral administration and persists for 24 hours. Chronic administration leads to sustained blood pressure control and may reverse some cardiac and vascular remodeling.

⚡ Pharmacokinetics Summary

Absorption: Losartan is well absorbed after oral administration in most species, with an oral bioavailability of approximately 33% in dogs due to first-pass metabolism. In cats, bioavailability is lower (around 25%). Food may decrease the rate but not the extent of absorption.

Distribution: Losartan is widely distributed, with a volume of distribution of approximately 0.5-1 L/kg. It crosses the blood-brain barrier poorly. Protein binding is high (98.7% in humans, similar in animals).

Metabolism: Losartan undergoes extensive first-pass metabolism in the liver, primarily by cytochrome P450 enzymes (CYP2C9 and CYP3A4) to form the active metabolite E-3174, which is 10-40 times more potent than losartan. The metabolism is species-dependent; dogs and cats produce E-3174 but at different rates.

Excretion: Losartan and its metabolites are excreted in both urine and feces. In dogs, approximately 35% of the dose is excreted in urine and 60% in feces. In cats, renal excretion is less significant. The elimination half-life of losartan is about 2 hours in dogs, but the active metabolite has a longer half-life (4-6 hours). In cats, the half-life is approximately 3-4 hours.

Half-Life: Dog: 2 hours (losartan), 4-6 hours (E-3174); Cat: 3-4 hours; Horse: 1.5-2 hours

Bioavailability: Dog: ~33%; Cat: ~25%; Horse: ~40%

Protein Binding: 98.7% (plasma proteins)

Available Formulations & Strengths

Oral Tablet 25 mg, 50 mg, 100 mg (PO)

Oral Solution (compounded) Various (e.g., 5 mg/mL) (PO)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to losartan or any component
Pregnancy (especially in the second and third trimesters) - risk of fetal toxicity
Concurrent use with aliskiren in patients with diabetes (not common in veterinary medicine)
Severe hepatic impairment (use with caution)
Bilateral renal artery stenosis (theoretical risk)

Warnings & Clinical Precautions:

Use with caution in patients with renal impairment; monitor renal function and potassium levels.
May cause hypotension, especially in volume-depleted animals.
Use with caution in animals with heart failure; monitor for worsening renal function.
Safety in pregnant or lactating animals has not been established; use only when clearly needed.
In cats, may cause gastrointestinal upset; administer with food to reduce vomiting.
Do not use in animals with known hypersensitivity to ARBs.
Monitor blood pressure regularly during therapy.
In food animals, withdrawal times must be observed; extra-label use requires veterinary oversight.

Adverse Effects & Reactions

Common:

Gastrointestinal upset (vomiting, diarrhea)
Hypotension
Hyperkalemia
Increased thirst or urination

Serious / Severe:

Acute kidney injury
Severe hypotension
Angioedema (rare)
Fetal toxicity in pregnant animals

Rare:

Hepatotoxicity
Blood dyscrasias
Anaphylaxis

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
ACE inhibitors (e.g., enalapril, benazepril)	Additive hypotensive effect and increased risk of hyperkalemia and renal dysfunction.	High
Potassium-sparing diuretics (e.g., spironolactone)	Increased risk of hyperkalemia.	High
NSAIDs (e.g., carprofen, meloxicam)	Reduced antihypertensive effect and increased risk of renal impairment.	Moderate
Diuretics (e.g., furosemide)	Enhanced hypotensive effect; risk of volume depletion and hypotension.	Moderate
Lithium	Increased lithium levels and toxicity.	Moderate
Potassium supplements	Increased risk of hyperkalemia.	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Hypotension
Tachycardia or bradycardia
Dizziness
Electrolyte imbalances (hyperkalemia)
Renal dysfunction

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Induce vomiting if recent ingestion and animal is stable. Administer activated charcoal to reduce absorption. Provide intravenous fluids to maintain blood pressure and renal perfusion. Monitor ECG and electrolytes. In severe cases, use vasopressors (e.g., dopamine) and treat hyperkalemia with calcium gluconate, insulin/glucose, or sodium bicarbonate. No specific antidote exists.

Food Animal Withdrawal Times

🥩 Meat: 0 days🥛 Milk: 0 days🥚 Eggs: 0 days

Losartan is not approved for use in food animals. Withdrawal times have not been established. Extra-label use in food animals requires a veterinary prescription and extended withdrawal periods should be considered. Consult FARAD for specific recommendations.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (20-25°C, 68-77°F)

Handling & Special Conditions: Keep container tightly closed. Protect from moisture. Compounded solutions should be stored according to the compounding pharmacy's instructions, typically refrigerated and used within a specified period.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not approved for veterinary use; approved for human use only.

Extra-Label (Off-Label) Use: Losartan is not FDA-approved for veterinary use. Its use in animals is extra-label (AMDUCA). For food animals, a valid veterinary-client-patient relationship is required, and withdrawal times must be extended. For non-food animals, extra-label use is permitted under veterinary supervision.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Losartan is an angiotensin II receptor blocker used in veterinary medicine primarily for the management of systemic hypertension and proteinuric chronic kidney disease, especially in dogs and cats. It is often considered an alternative to ACE inhibitors (e.g., enalapril) when cough or other adverse effects occur. In cats, losartan has been shown to reduce proteinuria and may be renoprotective. Dosing should be individualized based on blood pressure and renal parameters. Monitor serum creatinine, BUN, potassium, and blood pressure within 1-2 weeks after initiation or dose changes. Use with caution in animals with renal insufficiency or volume depletion. In horses and other species, use is limited and experimental. Always consider potential drug interactions, especially with ACE inhibitors and potassium-sparing diuretics. For food animals, extra-label use requires careful consideration of withdrawal times.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)