Aspirin

Aspirin, or acetylsalicylic acid, is a salicylate NSAID with analgesic, anti-inflammatory and antipyretic actions. At lower exposure it is also used to inhibit platelet function. Veterinary use has declined for many painful conditions because other agents offer more suitable species-specific evidence or tolerability.

Analgesic and antiplatelet uses involve different therapeutic objectives and exposure requirements. Platelet response is variable in animals, and species differences in salicylate elimination substantially affect persistence and accumulation.

Group

Salicylate NSAID; antiplatelet agent

Mechanism of Action

Aspirin irreversibly acetylates cyclooxygenase enzymes, reducing formation of prostaglandins from arachidonic acid. Lower prostaglandin production reduces inflammatory sensitization of pain pathways and contributes to analgesia, anti-inflammatory activity and antipyresis. Reduced protective prostaglandins also explain important gastrointestinal and renal effects.

In platelets, inhibition of COX-1 reduces thromboxane A2 formation and thereby limits platelet activation and aggregation. Because platelets cannot readily replace the acetylated enzyme, inhibition can persist beyond the presence of aspirin in plasma. The degree of suppression is nevertheless inconsistent among veterinary patients.

After absorption aspirin is hydrolyzed to salicylate, which contributes additional pharmacologic effects and is eliminated at markedly different rates between species. Aspirin’s platelet action is therefore distinct from simply selecting any NSAID, and an analgesic dose is not automatically an appropriate antithrombotic regimen.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Mild analgesia	10 mg/kg	PO	Every 12 h
Anti-inflammatory treatment	20–25 mg/kg	PO	Every 12 h
Antiplatelet treatment - typical amount	1–5 mg/kg	PO		Typical amount is listed without an explicit repeat interval; clinical benefit is inconsistent.
Antiplatelet treatment - higher-dose course	5–10 mg/kg	PO	Every 24–48 h	Clinical antiplatelet benefit is not consistently demonstrated.
Antiplatelet treatment - research regimen	0.5 mg/kg	PO	Every 12 h	Platelet aggregation impairment was observed in research dogs; clinical benefit is not established.
Anti-inflammatory treatment - frequent alternative	25 mg/kg	PO	Every 8 h
Antipyresis	10 mg/kg	PO	Every 12 h
IMHA antithrombotic adjunct	0.5 mg/kg	PO	Every 12 h	Low-dose adjunct; evaluate gastrointestinal and bleeding risk, especially with corticosteroids.
Glomerular disease antithrombotic adjunct	0.5 mg/kg	PO	Every 24 h
IMHA antithrombotic - daily alternative	0.5 mg/kg	PO	Every 24 h	Described with azathioprine and glucocorticoids; monitor bleeding.
Perioperative ocular analgesic - incomplete route	6.5 mg/kg	PO		Two or three times daily before elective intraocular surgery; route is not explicitly stated and surgical bleeding risk requires assessment.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Mild analgesia	10 mg/kg	PO	Every 48 h
Anti-inflammatory treatment	10–20 mg/kg	PO	Every 48 h
Antiplatelet treatment	80 mg/animal	PO	Every 48 h	No clinical efficacy study validates this fixed feline dose.
Antiplatelet treatment - 5 mg alternative	5 mg/animal	PO	Every 72 h	Reported alternative; no clinical study establishes efficacy.
Antiplatelet treatment - 80 mg alternative	80 mg/animal	PO	Every 72 h	Reported alternative; no clinical study establishes efficacy.
Analgesia - fixed tablet alternative	81 mg/animal	PO	Every 48–72 h
Arterial thromboembolism prophylaxis	40 mg/animal	PO	Every 72 h	Higher-dose historical alternative; clinical efficacy is uncertain.
Arterial thromboembolism prophylaxis - 81 mg alternative	81 mg/animal	PO	Every 72 h	Historical alternative; often less effective than other antithrombotic options.

Cattle

Use / stage	Dose	Route	Schedule	Instructions
Analgesic and anti-inflammatory treatment	100 mg/kg	PO	Every 12 h
High-dose historical listing	333 mg/kg	PO		Amount up to 333 mg/kg is described without a repeat interval; not a routine recommended course.

Sheep

Use / stage	Dose	Route	Schedule	Instructions
Analgesic and anti-inflammatory treatment	100 mg/kg	PO	Every 12 h

Goats

Use / stage	Dose	Route	Schedule	Instructions
Analgesic and anti-inflammatory treatment	100 mg/kg	PO	Every 12 h

Pigs

Use / stage	Dose	Route	Schedule	Instructions
Analgesic and anti-inflammatory treatment	10 mg/kg	PO	Every 6–8 h
Analgesia - frequent alternative	10 mg/kg	PO	Every 4 h

Horses

Use / stage	Dose	Route	Schedule	Instructions
Analgesic and anti-inflammatory treatment	25–50 mg/kg	PO	Every 12 h	Do not exceed 100 mg/kg per day. A total daily body-weight cap cannot be encoded as an amount-only max_daily_dose.
Analgesia - initial and maintenance course — Initial	25 mg/kg	PO	Every 12 h	Then reduce to 10 mg/kg daily.
Analgesia - initial and maintenance course — Maintenance	10 mg/kg	PO	Every 24 h
Analgesia - daily alternative	15–100 mg/kg	PO	Every 24 h
Laminitis antiplatelet adjunct	5–10 mg/kg	PO	Every 24–48 h
Laminitis antiplatelet - extended interval	20 mg/kg	PO	Every 96–120 h

Ferret

Use / stage	Dose	Route	Schedule	Instructions
Analgesia	10–20 mg/kg	PO	Every 24 h	Duration of effect can be short.

Rabbit

Use / stage	Dose	Route	Schedule	Instructions
Low-grade analgesia	5–20 mg/kg	PO	Every 24 h

Mouse

Use / stage	Dose	Route	Schedule	Instructions
Analgesia - historical small-rodent course	100–150 mg/kg	PO	Every 4 h	Species-specific historical regimen; do not extrapolate to dogs, cats or other mammals.

Rat

Use / stage	Dose	Route	Schedule	Instructions
Analgesia - historical small-rodent course	100–150 mg/kg	PO	Every 4 h	Species-specific historical regimen; do not extrapolate to dogs, cats or other mammals.

Gerbil

Use / stage	Dose	Route	Schedule	Instructions
Analgesia - historical small-rodent course	100–150 mg/kg	PO	Every 4 h	Species-specific historical regimen; do not extrapolate to dogs, cats or other mammals.

Hamster

Use / stage	Dose	Route	Schedule	Instructions
Analgesia - historical small-rodent course	100–150 mg/kg	PO	Every 4 h	Species-specific historical regimen; do not extrapolate to dogs, cats or other mammals.

Guinea pigs

Use / stage	Dose	Route	Schedule	Instructions
Analgesia - interval unspecified	87 mg/kg	PO		Repeat interval is not supplied.

Indications

Selected mild painful and inflammatory conditions, including historical osteoarthritis management.
Adjunctive anti-inflammatory treatment where aspirin is appropriate for the species and condition.
Selected antiplatelet treatment in patients at risk of platelet-associated thrombosis; response and clinical outcome evidence vary by indication.

Contraindications

Avoid in animals predisposed to gastrointestinal ulcers.
Use cautiously with coagulopathies.
Cats clear salicylates slowly and are especially susceptible to intoxication.
Heartworm-associated thromboembolism prevention lacks convincing benefit and may be inappropriate.
Aspirin hypersensitivity or active bleeding ulcers.
Use cautiously in asthma, renal or severe hepatic dysfunction, hypoalbuminemia and neonates.
Avoid during pregnancy, particularly late gestation; it can delay parturition.

Interactions

Corticosteroids increase gastrointestinal adverse-effect risk.
Other drugs that impair coagulation increase bleeding risk.
Other NSAIDs increase ulcer risk; allow an appropriate washout when changing therapy.
Can increase digoxin exposure or methotrexate toxicity; anticoagulants increase bleeding.
Urinary acidifiers can reduce salicylate elimination. Carbonic-anhydrase inhibitors can worsen systemic acidosis despite increasing urinary excretion.
Furosemide can delay elimination, phenobarbital can increase metabolism and spironolactone diuresis may be reduced.

Adverse effects

Vomiting, gastrointestinal ulceration and bleeding.
Platelet inhibition and increased bleeding tendency.

Toxicity

Salicylate poisoning can cause vomiting, hyperthermia, hyperventilation, acid–base disturbances, weakness, seizures and coma. Monitor blood gases, electrolytes, hydration and renal function. Decontamination, gastrointestinal protection and carefully monitored urinary alkalinization may be indicated; dialysis may be considered for severe poisoning.

Preparations

Preparation	Form	Strength	Route
Aspirin 81 mg tablet	Tablet	81 mg / tablet	PO
Aspirin 325 mg tablet	Tablet	325 mg / tablet	PO
Aspirin 3900 mg tablet	Tablet	3900 mg / tablet	PO
Aspirin 15600 mg tablet	Tablet	15600 mg / tablet	PO
Aspirin 31200 mg tablet	Tablet	31200 mg / tablet	PO
Durlaza extended-release 16.2 mg capsule Swallow intact; do not open or break.	Capsule	16.2 mg / capsule	PO

Monitoring and Administration

Monitor gastrointestinal signs and bleeding. Enteric-coated absorption in dogs can be erratic and incomplete; buffering does not prevent serious ulceration or perforation.

Aspirin can suppress measured thyroid hormone values after 2–4 weeks; values returned to baseline about 14 days after stopping in the described observation.

Keep tightly sealed and dry at room temperature; moisture causes hydrolysis. Extended-release Durlaza capsules must not be opened or broken.

For an avian drinking-water preparation, 5 g in 250 mL as the sole water source is described; exposure depends on intake. Because of hydrolysis, prepare fresh every 12 hours at room temperature or every 4 days refrigerated at 5°C. Monitor PCV and fecal blood when indicated; stop before planned surgery when clinically appropriate, with 1 week described for platelet recovery.

Routine aspirin after heartworm adulticide treatment lacks convincing clinical benefit and can be inappropriate; it is not a preferred prophylactic protocol. Use the metric strength of the exact bolus rather than an assumed grain conversion.