Nitroglycerin

Vasodilator Organic Nitrate

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog Topical (transdermal ointment) 0.25-1.0 inch (approximately 0.5-2 mg) per 10 kg body weight q6-8h Duration: As needed for acute management; taper to avoid tolerance
Notes: Apply to clipped, non-hairy area (e.g., inner ear, thorax) using gloves. Wear gloves to avoid human exposure. Rotate application sites to prevent skin irritation.
Cat Topical (transdermal ointment) 0.25-0.5 inch (approximately 0.5-1 mg) per cat q6-8h Duration: As needed for acute management; taper to avoid tolerance
Notes: Apply to inner ear or clipped area. Monitor for hypotension.
Horse Topical (transdermal ointment) 1-2 inches (approximately 2-4 mg) per horse q6-8h Duration: Short-term for acute laminitis
Notes: Use with caution; monitor for hypotension. Not a standard treatment.

Clinical Indications & Species Uses

General Indications
  • Vasodilation in acute and chronic congestive heart failure
  • Reduction of preload in pulmonary edema
  • Adjunctive therapy for myocardial ischemia
Dog (Canine)
  • Acute congestive heart failure (pulmonary edema)
  • Chronic congestive heart failure (adjunctive therapy)
  • Myocardial ischemia
  • Hypertensive crisis (off-label)
Cat (Feline)
  • Acute congestive heart failure (pulmonary edema)
  • Hypertrophic cardiomyopathy with congestive heart failure
  • Systemic hypertension (off-label)

Pharmacology & Mechanism of Action

Drug Class: Vasodilator | Pharmacological Group: Organic Nitrate

Mechanism of Action: Nitroglycerin is a prodrug that undergoes denitration to release nitric oxide (NO) via enzymatic and non-enzymatic pathways. NO activates soluble guanylyl cyclase in vascular smooth muscle cells, increasing cyclic guanosine monophosphate (cGMP) levels. Elevated cGMP activates protein kinase G, leading to dephosphorylation of myosin light chains and subsequent smooth muscle relaxation. This results in vasodilation, predominantly of venous capacitance vessels, reducing preload and myocardial oxygen demand. At higher doses, it also dilates coronary arteries and arterioles, improving collateral blood flow.

Pharmacodynamics: Nitroglycerin produces dose-dependent vasodilation. Low doses primarily dilate veins, reducing venous return and ventricular filling pressure (preload). Higher doses dilate arterioles, reducing systemic vascular resistance (afterload). In animals with congestive heart failure, it decreases pulmonary capillary wedge pressure and left ventricular end-diastolic pressure, alleviating pulmonary edema and improving cardiac output. It also relieves coronary vasospasm and may redistribute coronary blood flow to ischemic areas. Tolerance develops with continuous use due to depletion of sulfhydryl groups and reduced NO bioavailability.

⚡ Pharmacokinetics Summary

Absorption: Nitroglycerin is well absorbed through mucous membranes and skin. Oral administration undergoes extensive first-pass hepatic metabolism, resulting in low bioavailability (<10%). Transdermal and sublingual routes bypass first-pass metabolism, providing rapid absorption. In veterinary patients, transdermal ointment is commonly used for sustained absorption.
Distribution: Nitroglycerin is widely distributed throughout the body. It has a large volume of distribution due to its lipophilic nature. It crosses the placenta and is distributed into milk. Protein binding is approximately 60%.
Metabolism: Nitroglycerin is rapidly metabolized in the liver by glutathione-S-transferase and in vascular smooth muscle cells by mitochondrial aldehyde dehydrogenase. It is denitrated to inactive dinitrate and mononitrate metabolites. The liver is the primary site of metabolism, but extrahepatic metabolism occurs in the blood and vascular tissues.
Excretion: Metabolites are excreted primarily by the kidneys. Less than 1% of the parent drug is excreted unchanged in urine. Biliary excretion also occurs.
Half-Life: The half-life of nitroglycerin is very short, approximately 1-4 minutes in plasma. However, the pharmacodynamic effect may last longer due to active metabolites. In dogs, the half-life is approximately 2-3 minutes.
Bioavailability: Oral bioavailability is less than 10% due to first-pass metabolism. Sublingual and transdermal routes have higher bioavailability (approximately 40-50% for sublingual, variable for transdermal).
Protein Binding: Approximately 60% bound to plasma proteins.

Available Formulations & Strengths

Topical Ointment 2% (10 mg/g) (Topical)
Sublingual Tablet 0.3 mg, 0.4 mg, 0.6 mg (Sublingual)
Transdermal Patch 0.1 mg/hr, 0.2 mg/hr, 0.4 mg/hr, 0.6 mg/hr (Transdermal)
Intravenous Solution 5 mg/mL, 10 mg/mL (IV)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to nitroglycerin or other nitrates
  • Severe hypotension (systolic blood pressure < 90 mmHg)
  • Cardiac tamponade
  • Constrictive pericarditis
  • Concurrent use of phosphodiesterase-5 inhibitors (e.g., sildenafil) due to severe hypotension risk
  • Increased intracranial pressure
  • Severe anemia
Warnings & Clinical Precautions:
  • Use with caution in animals with hepatic or renal impairment
  • Monitor blood pressure and heart rate during administration
  • Tolerance may develop with continuous use; use intermittent dosing with nitrate-free intervals
  • Avoid sudden withdrawal after prolonged use to prevent rebound vasoconstriction
  • Wear gloves when applying topical ointment to prevent human exposure
  • Use with caution in animals with hypovolemia or dehydration
  • Not approved for food animals; extra-label use requires withdrawal time estimation

Adverse Effects & Reactions

Common:

  • Hypotension
  • Tachycardia (reflex)
  • Headache (in humans; not observed in animals)
  • Skin irritation at application site

Serious / Severe:

  • Severe hypotension
  • Syncope
  • Methemoglobinemia (rare, with high doses)
  • Worsening of myocardial ischemia due to reflex tachycardia

Rare:

  • Allergic reactions
  • Methemoglobinemia
  • Paradoxical bradycardia

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Phosphodiesterase-5 inhibitors (e.g., sildenafil, tadalafil) Additive vasodilation and severe hypotension High
Other vasodilators (e.g., hydralazine, ACE inhibitors) Additive hypotensive effects Moderate
Beta-blockers May reduce reflex tachycardia but can increase risk of hypotension and heart failure Moderate
Diuretics (e.g., furosemide) Additive reduction in preload; may cause excessive hypotension Moderate
Heparin Nitroglycerin may reduce heparin's anticoagulant effect Mild

Overdose & Toxicity Management

Signs of Toxicity:

  • Severe hypotension
  • Bradycardia or tachycardia
  • Syncope
  • Methemoglobinemia (cyanosis, dyspnea)
  • Seizures (in severe cases)

Emergency Treatment Protocol: Discontinue nitroglycerin immediately. Provide supportive care: place animal in recumbent position, administer IV fluids to maintain blood pressure, and use vasopressors (e.g., dopamine, norepinephrine) if needed. For methemoglobinemia, administer methylene blue (1-2 mg/kg IV) as an antidote. Monitor vital signs and oxygen saturation. In severe cases, consider activated charcoal if recent oral ingestion, but most veterinary exposures are topical or IV.

Food Animal Withdrawal Times

Nitroglycerin is not approved for use in food animals. If used extra-label, a withdrawal time of at least 30 days for meat and 7 days for milk is recommended, but consult with a veterinary pharmacologist for specific guidance.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature (20-25°C, 68-77°F). Protect from moisture.

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Keep container tightly closed. Store in original container to protect from light. Do not freeze. Avoid exposure to heat.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Nitroglycerin is FDA-approved for human use. No veterinary-approved formulations exist; use in animals is extra-label.

Extra-Label (Off-Label) Use: In the US, extra-label use of nitroglycerin in food animals is prohibited under AMDUCA if an approved animal drug exists that is labeled for the condition. Since no approved veterinary formulation exists, extra-label use is allowed only under veterinary supervision with appropriate withdrawal times. In companion animals, extra-label use is common.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Nitroglycerin is primarily used in veterinary medicine as an adjunctive treatment for acute congestive heart failure, particularly pulmonary edema, in dogs and cats. It is most effective when used early in the management of heart failure, along with furosemide and oxygen therapy. The transdermal ointment is the most common formulation used in veterinary practice due to ease of administration and sustained effect. However, its use has declined with the advent of other vasodilators like ACE inhibitors and pimobendan. Tolerance is a significant issue with continuous use; therefore, intermittent dosing with nitrate-free intervals (e.g., 8-12 hours off) is recommended. Blood pressure and heart rate should be monitored closely, especially in cats, which are sensitive to hypotension. Topical application should be done with gloves to prevent human exposure. Nitroglycerin should not be used as a sole therapy for heart failure but as part of a comprehensive treatment plan.

References & Literature

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