Aspirin (Acetylsalicylic Acid)
Aspirin, also known as acetylsalicylic acid, is the acetic ester of salicylic acid. It appears as a white crystalline powder or as tabular or needle-like crystals, exhibiting weak acidity. It is sparingly soluble in water but readily soluble in ethanol.
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Storage
Aspirin tablets should be stored in a dry, tightly closed container.
They must not be used if past the expiration date or if they emit a vinegar-like odor, which indicates decomposition.
Aspirin is stable in dry air, but in the presence of moisture or water it hydrolyzes into acetic acid and salicylic acid, producing a strong acetic smell.
Heating accelerates this hydrolysis.
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Pharmacology / Indications / Pharmacokinetics
Aspirin inhibits cyclooxygenase (COX), the enzyme responsible for prostaglandin and thromboxane synthesis, thereby producing antipyretic, analgesic, anti-inflammatory, and antiplatelet effects.
Most cells can resynthesize COX, but platelets cannot, making aspirin’s inhibition of platelet aggregation irreversible.
Aspirin is used in all animal species as an analgesic and antipyretic. It is one of the few NSAIDs considered relatively safe for both dogs and cats, though it can cause significant gastrointestinal bleeding in dogs.
In addition to its antipyretic, analgesic, and anti-inflammatory effects, aspirin inhibits platelet aggregation and may therefore be used to treat:
•Disseminated intravascular coagulation (DIC) in dogs
•Pulmonary arterial disease secondary to heartworm infection
•Myocarditis in cats
At low doses, due to its antiplatelet and anti-inflammatory properties, it can support therapy in glomerular disease.
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Drug Safety
Aspirin is contraindicated in animals with a history of hypersensitivity to the drug or with bleeding ulcers.
Use with caution in animals with bleeding disorders, asthma, or renal insufficiency.
Because aspirin is highly protein-bound, animals with hypoproteinemia should receive lower doses to avoid toxicity.
Caution is also advised in severe liver or kidney dysfunction, as aspirin may elevate laboratory indicators.
Due to its irreversible effect on platelets, it is recommended to discontinue use one week before surgery whenever possible.
Cats metabolize and excrete salicylic acid very slowly, making them highly susceptible to toxicity if dosing is not carefully monitored.
Young animals are also more sensitive — adult doses may cause poisoning in juveniles.
Aspirin is potentially teratogenic and has been reported to prolong labor; avoid use during pregnancy, especially in the late gestation period.
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Toxicity and Overdose
Signs in dogs and cats include:
•Depression
•Vomiting (possibly with blood)
•Loss of appetite
•Hyperthermia and rapid breathing
Initially, excessive respiration may cause respiratory alkalosis, followed by metabolic acidosis.
If untreated, animals may develop muscle relaxation, pulmonary and cerebral edema, hypernatremia, hypokalemia, ataxia, seizures, coma, and death.
Treatment of acute overdose (within 12 hours):
•Gastric lavage
•Administration of activated charcoal and oral cathartics
•IV fluids and continuous monitoring (e.g., blood gases)
•Oral or gastric 3–5% sodium bicarbonate to reduce absorption
•5% glucose IV to relieve dehydration
•Forced alkaline diuresis with sodium bicarbonate to enhance excretion
•Mannitol (1–2 mg/kg/hr) may promote diuresis
•Diazepam IV for seizure control
•Vitamin K (2.5 mg/kg every 8–12 h) and Vitamin C (25 mg, non-oral) for hypoprothrombinemia (note: vitamin C may counteract urine alkalinization)
•In severe cases: peritoneal dialysis or hemodialysis
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Drug Interactions
•Urine alkalinizers (e.g., acetazolamide, sodium bicarbonate) increase salicylate excretion.
•Acidifying agents (e.g., methionine, ammonium chloride, vitamin C) reduce renal excretion.
•Furosemide competes with aspirin for renal excretion, prolonging aspirin’s half-life and increasing toxicity risk.
•Phenobarbital induces hepatic enzymes, accelerating aspirin metabolism.
•Corticosteroids increase salicylate clearance but enhance GI ulcer risk when combined with aspirin or other NSAIDs.
•Heparin and oral anticoagulants: increased bleeding risk.
•Aspirin inhibits spironolactone’s diuretic action.
•Tetracyclines should not be given simultaneously or within 1 hour of buffered aspirin due to chelation with antacids.
•Digoxin: aspirin inhibits elimination, increasing plasma concentration.
•Aminoglycosides: may increase nephrotoxicity (not fully proven). Use only with careful benefit–risk assessment.
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Dosage
Dogs
Recommended to use buffered formulations of aspirin.
Analgesic:
•10–25 mg/kg PO every 8–12 hours
•10–20 mg/kg PO every 12 hours
•10–25 mg/kg PO mixed with food every 12 hours
•10 mg/kg PO every 12 hours
Anti-inflammatory / Antirheumatic:
•25 mg/kg PO every 8 hours
Antipyretic:
•10 mg/kg PO twice daily
Post-adulticide therapy for heartworm disease:
•7–10 mg/kg PO once daily
Antiplatelet / Antithrombotic therapy:
•0.5 mg/kg PO twice daily
•For glomerular disease support: 0.5 mg/kg PO every 12–24 hours
•Adjunct in immune-mediated hemolytic anemia (with azathioprine and corticosteroids): 0.5 mg/kg PO once daily
For DIC:
•150–300 mg per 20 kg PO once daily or every other day for 10 days
Preoperative analgesic/anti-inflammatory (ophthalmic surgery):
•6.5 mg/kg PO twice or three times daily
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Source: Veterinary Drug Manual
Compiled and published by: Mao Pipipet Veterinary Team