Aspirin
Acetylsalicylic Acid
Also known as: ASA · Bayer · Ecotrin · Bufferin · Ascriptin · Alka-Seltzer · Empirin · Halfprin · Aspirin BP · Acetylsalicylic acid · Acidum acetylsalicylicum · Salicylic acid acetate · Acetylsal acid · Polopiryna
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| For analgesia | 10 mg/kg PO q12h | PO | q12h | — | 🌎 NA |
| As an antiinflammatory/antirheumatic | 25 mg/kg PO q8h | PO | q8h | — | 🌎 NA |
| For antipyrexia | 10 mg/kg PO twice daily | PO | twice daily | — | 🌎 NA |
| Post-Adulticide therapy for heartworm disease | 7-10 mg/kg PO once a day | PO | once a day | — | 🌎 NA |
| For adjunctive therapy in IMHA (antithrombotic) | 0.5 mg/kg PO twice daily | PO | twice daily | — | 🌎 NA |
| For adjunctive therapy of glomerular disease (antithrombotic) | 0.5 mg/kg PO q24h | PO | q24h | — | 🌎 NA |
| For adjunctive therapy with azathioprine and glucocorticoids for immune-mediated hemolytic anemia | 0.5 mg/kg PO once daily | PO | once daily | — | 🌎 NA |
| As an analgesic/antiinflammatory prior to elective intraocular surgery | 6.5 mg/kg two to three times daily | PO | two to three times daily | — | 🌎 NA |
| Reduction of platelet aggregation (e.g. IMHA) | 0.5-1 mg/kg | PO | q24h | — | 🌍 EU |
| Analgesia, pyrexia, inflammation | 10-20 mg/kg | PO | q12h | — | 🌍 EU |
- For analgesia: Recommend using buffered varieties of aspirin in dogs.
- For adjunctive therapy in IMHA (antithrombotic): At this low dose risk for gastric ulceration is low, but adding misoprostol may reduce the risk.
- Reduction of platelet aggregation (e.g. IMHA): Alternatively 0.5 mg/kg p.o. q12h. Doses are anecdotal.
- Analgesia, pyrexia, inflammation: Safety and efficacy of this dose has not been established.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| For analgesia/antipyrexia/antiinflammatory | One 'baby' aspirin (81 mg) PO every 2-3 days | PO | every 2-3 days | — | 🌎 NA |
| For analgesia/antipyrexia/antiinflammatory | 10 mg/kg PO every other day | PO | every other day | — | 🌎 NA |
| For analgesia/antipyrexia/antiinflammatory | 10 mg/kg PO q48-72h in food | PO | q48-72h | — | 🌎 NA |
| For prophylaxis of arterial thromboembolism (ATE) | 5 mg (total dose) per cat PO q72hours (every 3rd day) | PO | q72h | — | 🌎 NA |
| For prophylaxis of arterial thromboembolism | 81 mg (total dose; one 'baby' aspirin) q72hours (every 3rd day) | PO | q72h | — | 🌎 NA |
| For prevention of thromboembolism (High dose) | 40 mg per cat PO q72h | PO | q72h | — | 🌎 NA |
| For prevention of thromboembolism (Low dose) | 5 mg per cat PO q72h | PO | q72h | — | 🌎 NA |
| Reduction of platelet aggregation (low dose) | 18.75 mg (1/4 of a 75 mg tablet) per average sized cat | PO | 3 days a week | — | 🌍 EU |
| Reduction of platelet aggregation (high dose) | 75 mg per average sized cat | PO | 3 days a week | — | 🌍 EU |
| Reduction of platelet aggregation (very low dose) | 0.5 mg/kg | PO | q24h | — | 🌍 EU |
- For prophylaxis of arterial thromboembolism: Likely a weaker, but less expensive option than clopidogrel/LMWH. Recommended in all cats with atrial enlargement and cardiomyopathy.
- Reduction of platelet aggregation (low dose): Safety and efficacy not fully evaluated.
- Reduction of platelet aggregation (high dose): May be associated with a higher risk of GI side effects.
- Reduction of platelet aggregation (very low dose): Suggested to inhibit platelet COX without preventing beneficial prostacyclin production.
Small Mammals
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| For low grade analgesia (Rabbits) | 5-20 mg/kg PO once daily | PO | once daily | — | 🌎 NA |
| General use (Mice, Rats, Gerbils, Hamsters) | 100-150 mg/kg PO q4h | PO | q4h | — | 🌎 NA |
| General use (Guinea pigs) | 87 mg/kg PO | PO | once | — | 🌎 NA |
Ferrets
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| General use | 10-20 mg/kg PO once daily | PO | once daily | — | 🌎 NA |
- General use: Has short duration of activity in ferrets.
Birds
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| General use | 5 grams in 250 mL of water as sole water source | PO | continuous | — | 🌎 NA |
- General use: Because of significant hydrolysis, solution should be freshly prepared every 12 hours if stored at room temperature or every 4 days if refrigerated at 5°C.
Horses
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| For analgesia (Mature Horses) | two to four 240 grain boluses PO | PO | once | — | 🌎 NA |
| For analgesia (Foals) | one to two 240 grain boluses | PO | once | — | 🌎 NA |
| For analgesia | 25 mg/kg PO q12h initially, then 10 mg/kg once daily | PO | q12h initially, then once daily | — | 🌎 NA |
| For analgesia | 15-100 mg/kg PO once daily | PO | once daily | — | 🌎 NA |
| For anti-platelet activity as an adjunctive treatment of laminitis | 5-10 mg/kg PO q24-48 hours or 20 mg/kg PO every 4-5 days | PO | q24-48h or every 4-5 days | — | 🌎 NA |
- For analgesia (Mature Horses): Allow animals to drink water after administration.
- For analgesia (Foals): Allow animals to drink water after administration.
Cattle
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| For analgesia/antipyrexia | 100 mg/kg PO q12h | PO | q12h | — | 🌎 NA |
| For analgesia/antipyrexia (Mature Cattle) | two to four 240 grain boluses PO | PO | once | — | 🌎 NA |
| For analgesia/antipyrexia (Calves) | one to two 240 grain boluses | PO | once | — | 🌎 NA |
- For analgesia/antipyrexia (Mature Cattle): Allow animals to drink water after administration.
- For analgesia/antipyrexia (Calves): Allow animals to drink water after administration.
Swine
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| For analgesia | 10 mg/kg q4h PO | PO | q4h | — | 🌎 NA |
| For analgesia | 10 mg/kg q6h PO | PO | q6h | — | 🌎 NA |
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Aspirin (Acetylsalicylic Acid) is a classic nonsteroidal anti-inflammatory drug (NSAID) utilized across multiple veterinary species for its analgesic, antipyretic, and anti-inflammatory properties.
Beyond pain management, aspirin is uniquely valuable in veterinary medicine for its antiplatelet effects. It is frequently employed as an antithrombotic agent in dogs (e.g., for immune-mediated hemolytic anemia, heartworm disease, or glomerular disease) and in cats (e.g., for the prevention of arterial thromboembolism secondary to hypertrophic cardiomyopathy).
Clinical Pearl: Aspirin must be used with extreme caution in cats. Felines have a relative deficiency in glucuronyl transferase, the hepatic enzyme responsible for metabolizing salicylates. This results in a drastically prolonged half-life (up to 45 hours in cats compared to ~8 hours in dogs), making them highly susceptible to fatal accumulation and toxicity if dosed too frequently.
Mechanism of action
Aspirin exerts its effects primarily through the irreversible inhibition of cyclooxygenase (COX) enzymes, with a strong affinity for COX-1.
- Anti-inflammatory & Analgesic: Inhibits COX-1 (and to a lesser extent COX-2) → decreases synthesis of pro-inflammatory prostaglandins.
- Antiplatelet Effect: Irreversibly acetylates COX-1 in platelets → blocks the synthesis of Thromboxane A2 (TXA2), a potent inducer of platelet aggregation and vasoconstriction. Because platelets lack a nucleus, they cannot synthesize new COX enzymes; thus, the antiplatelet effect lasts for the lifespan of the platelet (typically 7-10 days).
- Gastric Protection Modulator: While aspirin does not directly inhibit COX-2, it modifies it to interact with lipoxygenase (LOX) → produces aspirin-triggered lipoxin (ATL), which provides some gastric mucosal protective actions, potentially explaining why gastric damage may decrease with chronic use.
Safety & warnings
Contraindications
- Known hypersensitivity to salicylates
- Active gastrointestinal bleeding or ulcers
- Bleeding disorders (relative contraindication)
- Asthma (relative contraindication)
- Renal insufficiency (relative contraindication)
- Pregnancy (especially later stages; low-grade teratogen and may delay labor)
Adverse effects
- Gastric irritation (nausea, anorexia, vomiting)
- Gastrointestinal ulceration and occult blood loss
- Secondary anemia or hypoproteinemia (due to chronic GI blood loss)
- Metabolic acidosis (especially in cats)
- Hypersensitivity reactions (rare in dogs)
Precautions
Hepatic and Renal Function: Use with extreme caution and enhanced monitoring in patients with severe hepatic failure or diminished renal function.
Hypoalbuminemia: Because aspirin is highly protein-bound, patients with low albumin may require lower dosages to prevent toxicity from increased free drug levels.
Surgical Procedures: Due to irreversible platelet inhibition, aspirin therapy should ideally be halted one week prior to elective surgical procedures to prevent excessive bleeding.
Species Specifics: Must be used with extreme caution in cats due to their inability to rapidly metabolize salicylates. Use cautiously in neonatal animals, as adult doses may lead to toxicity.
Drug interactions
Significantly increases the renal excretion of salicylates; carbonic anhydrase inhibitors may cause systemic acidosis and increase CNS salicylate levels, leading to toxicity.
May increase the likelihood of nephrotoxicity.
May increase salicylate clearance (decreasing serum levels) and significantly increase the risk of gastrointestinal ulceration and bleeding.
Aspirin may increase plasma levels of digoxin by decreasing its clearance.
May compete with renal excretion of aspirin, delaying its elimination and potentially causing toxicity at high doses.
Increased risk of bleeding due to synergistic antiplatelet/anticoagulant effects.
Aspirin may displace methotrexate from plasma proteins, increasing the risk of methotrexate toxicity.
Increased risk of severe GI ulceration. A washout period of 3-10 days is recommended when switching from aspirin to a COX-2 selective NSAID.
May increase the rate of aspirin metabolism by inducing hepatic enzymes.
Aspirin may antagonize the uricosuric effects of these drugs.
Aspirin may inhibit the diuretic activity of spironolactone.
Antacids in buffered aspirin may chelate tetracyclines; separate doses by at least one hour.
Can decrease the urinary excretion of salicylates, potentially leading to accumulation.
Increased risk of GI ulceration. Do not administer concurrently (Exception: specific protocols for IMHA treatment).
Monitoring
- Analgesic and/or antipyretic efficacy
- Bleeding times (if indicated)
- Packed Cell Volume (PCV)
- Stool guaiac tests (for occult GI bleeding)
- Acid-base status (in cases of overdose)
Pharmacokinetics
Half-life
Absorption
Rapidly absorbed from the stomach and proximal small intestine in monogastric animals. Absorption is slow from the GI tract in cattle, but approximately 70% of an oral dose is absorbed.
Distribution
Widely distributed throughout the body. Highest levels in liver, heart, lungs, renal cortex, and plasma. Highly protein bound (90% at lower concentrations, 70% at higher concentrations). Crosses the placenta and is excreted in milk.
Metabolism
Metabolized in the liver primarily by conjugation with glycine and glucuronic acid via glucuronyl transferase. Cats are deficient in this pathway, leading to prolonged half-lives and susceptibility to accumulation.
Elimination
Rapidly excreted by the kidneys via both filtration and renal tubular secretion. Excretion is highly pH dependent and can be significantly increased by raising urine pH to 5-8.
Overdose
Clinical Signs of Acute Toxicity:
- Early signs: Depression, vomiting (may be blood-tinged), anorexia, hyperthermia, panting/increased respiratory rate.
- Acid-Base Disturbances: Initially, a respiratory alkalosis occurs due to hyperventilation, followed by a profound metabolic acidosis.
- Severe signs: Muscular weakness, pulmonary and cerebral edema, hypernatremia, hypokalemia, ataxia, seizures, coma, and death.
Treatment Protocol:
- Decontamination: Emesis (if within 12 hours of ingestion), activated charcoal, and an oral cathartic. Gastric lavage with 3-5% sodium bicarbonate may delay absorption.
- Fluid Therapy: IV fluids (e.g., Dextrose 5% in water) to correct dehydration.
- Alkaline Diuresis: Forced alkaline diuresis with sodium bicarbonate enhances renal excretion, but should only be attempted if acid-base status can be monitored. Mannitol (1-2 g/kg/hr) may enhance diuresis.
- Supportive Care: GI protectants. Control seizures with IV diazepam.
- Coagulopathy Management: Phytonadione (Vitamin K1) at 2.5 mg/kg divided q8-12h and ascorbic acid (though ascorbic acid may negate urinary alkalinization).
- Heroic Measures: Peritoneal dialysis or exchange transfusions in severe cases.
Available products
Formulations
- Tablets (plain, enteric-coated, chewable, buffered)
- Extended-release tablets
- Boluses (240 grain, 480 grain)
- Oral gel
- Powder
- Granules
- Liquid concentrate
Veterinary
- Aspirin Tablets (Enteric-Coated): 81 mg
- Aspirin Tablets (Buffered, Microencapsulated, Chewable for dogs): 150 mg & 450 mg
- Aspirin Tablets 60 grain (3.9 g)
- Aspirin Boluses 240 grain (15.6 g)
- Aspirin Boluses 480 grain (31.2 g)
- Oral Aspirin Gel: 250 mg/mL and 500 mg/mL
- Aspirin Powder: 50% acetylsalicylic acid in base
- Aspirin Granules: 2.5 gram per 39 mL scoop
- Aspirin Liquid Concentrate (equiv to 12% aspirin)
Human-labeled
- Aspirin Chewable Tablets: 81 mg
- Aspirin Tablets (plain uncoated): 325 mg & 500 mg
- Aspirin Tablets (enteric coated): 81 mg, 165 mg, 325 mg, 500 mg, 650 mg, & 800 mg
- Aspirin Extended-controlled Release Tablets: 81 mg, 650 mg, 800 mg & 975 mg
- Aspirin Tablets (buffered uncoated): 325 mg & 500 mg
- Aspirin Tablets (buffered coated): 325 mg & 500 mg
Regulatory status
Available as human generic (Aspirin BP) and component of many others.
Available as human generic (Aspirin BP) and component of many others.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store tablets in tight, moisture-resistant containers. Aspirin is stable in dry air but readily hydrolyzes to acetate and salicylate when exposed to water or moist air. Do not use if a strong vinegar-like odor is noted, as this indicates degradation.
Client information
- Watch for Bleeding: Contact your veterinarian immediately if you notice signs of stomach ulcers or bleeding, such as black, tarry stools, vomit that looks like coffee grounds, loss of appetite, or severe lethargy.
- Strict Dosing: Never give more aspirin than prescribed, and never give it more frequently. This is especially critical for cats, as they process aspirin very slowly and can easily be fatally poisoned by standard human doses.
- Food Animal Withdrawal: While there are no official FDA withdrawal times, a minimum of 1 day withdrawal for milk or meat is recommended to protect salicylate-sensitive individuals.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.