Aspirin

Nonsteroidal Anti-inflammatory Drug (NSAID) Salicylate

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 10-25 mg/kg q8-12h Duration: As needed for pain/inflammation; for antiplatelet, 5-10 mg/kg q24-48h
Notes: Use with food to reduce GI upset. Monitor for GI bleeding.
Cat PO 10-20 mg/kg (but often lower, e.g., 5-10 mg/kg) q48-72h Duration: Short-term only; due to prolonged half-life
Notes: Extreme caution; cats have slow metabolism. Use only if no alternative.
Horse PO 25-100 mg/kg q12h Duration: As needed
Notes: Monitor for GI ulceration.
Cattle PO 100 mg/kg q12h Duration: As needed
Notes: Not commonly used; other NSAIDs preferred.
Small Ruminants PO 100 mg/kg q12h Duration: As needed
Notes: Limited data; use with caution.
Rabbit PO 100 mg/kg q12-24h Duration: Short-term
Notes: Limited data; monitor for GI stasis.

Clinical Indications & Species Uses

General Indications
  • Analgesia
  • Anti-inflammatory
  • Antipyretic
  • Antiplatelet
Dog (Canine)
  • Pain and inflammation associated with musculoskeletal disorders
  • Fever reduction
  • Antiplatelet therapy (e.g., prevention of thromboembolism)
Cat (Feline)
  • Pain and inflammation (use with caution due to prolonged half-life)
  • Antiplatelet therapy (e.g., prevention of arterial thromboembolism)
Horse (Equine)
  • Pain and inflammation (e.g., laminitis, musculoskeletal injuries)
  • Fever reduction
Cattle (Bovine)
  • Pain and inflammation (e.g., mastitis, respiratory disease)
  • Fever reduction
Small Ruminants (Sheep / Goat)
  • Pain and inflammation (e.g., lameness, mastitis)
  • Fever reduction
Rabbit & Small Mammals
  • Pain and inflammation (use with caution; limited data)
Avian & Poultry
  • Pain and inflammation (limited data; use with caution)
Exotic & Other Species
  • Pain and inflammation in some exotic species (limited data)

Pharmacology & Mechanism of Action

Drug Class: Nonsteroidal Anti-inflammatory Drug (NSAID) | Pharmacological Group: Salicylate

Mechanism of Action: Aspirin irreversibly inhibits cyclooxygenase (COX-1 and COX-2) enzymes by acetylating a serine residue in the active site, thereby blocking the conversion of arachidonic acid to prostaglandins, prostacyclin, and thromboxanes. This results in anti-inflammatory, analgesic, antipyretic, and antiplatelet effects. The antiplatelet effect is due to inhibition of thromboxane A2 synthesis in platelets, which lasts for the lifespan of the platelet because platelets lack the ability to synthesize new COX enzymes.

Pharmacodynamics: Aspirin produces dose-dependent effects: low doses primarily inhibit platelet aggregation, while higher doses provide anti-inflammatory and analgesic effects. It reduces fever by acting on the hypothalamic heat-regulating center. Anti-inflammatory effects are mediated by inhibition of prostaglandin synthesis at sites of inflammation. In animals, aspirin also has effects on gastrointestinal mucosa, renal function, and hemostasis.

⚑ Pharmacokinetics Summary

Absorption: Aspirin is well absorbed from the gastrointestinal tract after oral administration. Absorption is rapid in monogastric animals, but in ruminants, it may be slower due to ruminal degradation. Food can delay absorption but not the extent.
Distribution: Aspirin is widely distributed throughout the body. It crosses the blood-brain barrier and placenta. It is highly bound to plasma proteins (approximately 50-80% depending on species). It distributes into synovial fluid, cerebrospinal fluid, and milk.
Metabolism: Aspirin is rapidly hydrolyzed to salicylic acid in the liver and plasma by esterases. Salicylic acid is then conjugated with glycine to form salicyluric acid, and with glucuronic acid to form salicyl acyl glucuronide and salicyl phenolic glucuronide. In cats, metabolism is slower due to deficient glucuronidation capacity, leading to prolonged half-life.
Excretion: Salicylates are excreted primarily by the kidneys via glomerular filtration and tubular secretion. Excretion is pH-dependent; alkaline urine increases ionization and excretion. In dogs, approximately 10-20% is excreted unchanged, while the rest as metabolites.
Half-Life: Dog: 7.5-8 hours; Cat: 22-45 hours; Horse: 1-2 hours; Cattle: 0.5-1 hour; Pig: 1-2 hours.
Bioavailability: Oral bioavailability is high (nearly 100%) in monogastric animals, but may be reduced in ruminants due to ruminal degradation.
Protein Binding: Dose-dependent; approximately 50-80% bound to albumin in dogs and cats.

Available Formulations & Strengths

Oral Tablet 81 mg, 325 mg, 500 mg (PO)
Oral Powder Various (PO)
Oral Solution Various (PO)
Injectable Solution (not commonly used in veterinary medicine) Various (IV/IM)

Contraindications & Clinical Warnings

Contraindications:
  • Known hypersensitivity to salicylates
  • Active gastrointestinal ulceration or bleeding
  • Bleeding disorders (e.g., thrombocytopenia, hemophilia)
  • Concurrent use of other NSAIDs or corticosteroids
  • Pregnancy (especially third trimester) unless benefits outweigh risks
  • Renal or hepatic insufficiency
  • Cats with severe debilitation or dehydration
Warnings & Clinical Precautions:
  • Use with caution in animals with pre-existing GI disease, renal disease, or hepatic disease.
  • Cats are particularly sensitive due to slow metabolism; avoid repeated dosing.
  • May cause gastric ulceration and bleeding, especially at high doses or with prolonged use.
  • Monitor for signs of bleeding (e.g., melena, hematemesis).
  • Use with caution in dehydrated or hypovolemic animals due to risk of renal toxicity.
  • In horses, may cause right dorsal colitis with prolonged use.
  • Avoid use in animals with asthma or respiratory disease.
  • Do not use in neonates or very young animals due to risk of Reye's syndrome-like effects.

Adverse Effects & Reactions

Common:

  • Gastrointestinal upset (vomiting, diarrhea, anorexia)
  • Gastric ulceration
  • Increased bleeding time

Serious / Severe:

  • Gastrointestinal perforation
  • Renal toxicity (especially in dehydrated animals)
  • Hepatotoxicity
  • Bone marrow suppression
  • Metabolic acidosis (in overdose)

Rare:

  • Hypersensitivity reactions (urticaria, anaphylaxis)
  • Reye's syndrome-like encephalopathy in young animals
  • Salicylate toxicity (tinnitus, hyperthermia, respiratory alkalosis then acidosis)

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Other NSAIDs (e.g., carprofen, meloxicam) Increased risk of GI ulceration and renal toxicity High
Corticosteroids Increased risk of GI ulceration and bleeding High
Anticoagulants (e.g., warfarin, heparin) Increased risk of bleeding High
Furosemide Reduced diuretic effect and increased risk of renal toxicity Moderate
ACE inhibitors (e.g., enalapril) Reduced antihypertensive effect and increased risk of renal dysfunction Moderate
Methotrexate Increased methotrexate toxicity due to reduced renal clearance High

Overdose & Toxicity Management

Signs of Toxicity:

  • Vomiting
  • Hyperthermia
  • Tachypnea
  • Respiratory alkalosis
  • Metabolic acidosis
  • Depression
  • Seizures
  • Coma
  • Gastrointestinal bleeding
  • Renal failure

Emergency Treatment Protocol: Induce emesis if recent ingestion and animal is conscious. Administer activated charcoal. Provide supportive care: IV fluids, correction of acid-base and electrolyte imbalances, cooling for hyperthermia. Sodium bicarbonate may be used to alkalinize urine and enhance salicylate excretion. In severe cases, hemodialysis or peritoneal dialysis may be considered. Monitor for GI bleeding and renal function.

Food Animal Withdrawal Times

πŸ₯© Meat: 0 daysπŸ₯› Milk: 0 daysπŸ₯š Eggs: 0 days

Aspirin is not approved for use in food animals in many countries; however, if used extra-label, withdrawal times should be determined based on residue studies. In the US, no official withdrawal times are established; consult FARAD (Food Animal Residue Avoidance Databank) for guidance.

Storage, Handling & Regulatory Information

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

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

Dispensing Status: Over-The-Counter (OTC)

Approval Status: Aspirin is not FDA-approved for veterinary use in dogs, cats, or horses, but it is approved for use in humans. Some formulations may be approved for use in food animals (e.g., bolus for cattle) but are not commonly used.

Extra-Label (Off-Label) Use: In the US, aspirin is available over-the-counter for human use, but for veterinary use it may be used extra-label under AMDUCA (Animal Medicinal Drug Use Clarification Act) provided a valid veterinarian-client-patient relationship exists. For food animals, extra-label use is prohibited if an approved animal drug is available that contains the same active ingredient in the same dosage form and is labeled for the same species and indication.

Clinical Pearls & Practice Notes

πŸ’‘ Clinical Insights: Aspirin is a non-selective NSAID that provides analgesic, anti-inflammatory, and antipyretic effects. It is commonly used in dogs for pain and inflammation, and at low doses for antiplatelet therapy. In cats, its use is limited due to prolonged half-life and risk of toxicity; if used, dosing intervals must be extended. In horses, it is used for laminitis and musculoskeletal pain, but other NSAIDs like phenylbutazone are often preferred. Aspirin should be used with caution in animals with GI, renal, or hepatic disease. It is not recommended for long-term use due to the risk of GI ulceration. For antiplatelet effects, low doses are used, and monitoring of bleeding times may be considered. In food animals, use is limited due to lack of withdrawal times and availability of safer alternatives. Always consider the individual animal's health status and potential drug interactions before prescribing aspirin.

References & Literature

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