Aspirin
**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: 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) โ decreases synthesis of pro-inflammatory **prostaglandins**. - **Antiplatelet Effect:** Irreversibly acetylates COX-1 in platelets โ 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) โ produces **aspirin-triggered lipoxin (ATL)**, which provides some gastric mucosal protective actions, potentially explaining why gastric damage may decrease with chronic use.
Dosing by species
- For analgesia ยท 10 mg/kg PO q12h ยท PO ยท q12h ยท Recommend using buffered varieties of aspirin in dogs.
- As an antiinflammatory/antirheumatic ยท 25 mg/kg PO q8h ยท PO ยท q8h
- For antipyrexia ยท 10 mg/kg PO twice daily ยท PO ยท twice daily
- Post-Adulticide therapy for heartworm disease ยท 7-10 mg/kg PO once a day ยท PO ยท once a day
- For adjunctive therapy in IMHA (antithrombotic) ยท 0.5 mg/kg PO twice daily ยท PO ยท twice daily ยท At this low dose risk for gastric ulceration is low, but adding misoprostol may reduce the risk.
- For adjunctive therapy of glomerular disease (antithrombotic) ยท 0.5 mg/kg PO q24h ยท PO ยท q24h
- For adjunctive therapy with azathioprine and glucocorticoids for immune-mediated hemolytic anemia ยท 0.5 mg/kg PO once daily ยท PO ยท once daily
- As an analgesic/antiinflammatory prior to elective intraocular surgery ยท 6.5 mg/kg two to three times daily ยท PO ยท two to three times daily
- Reduction of platelet aggregation (e.g. IMHA) ยท 0.5-1 mg/kg ยท PO ยท q24h ยท Alternatively 0.5 mg/kg p.o. q12h. Doses are anecdotal.
- Analgesia, pyrexia, inflammation ยท 10-20 mg/kg ยท PO ยท q12h ยท Safety and efficacy of this dose has not been established.
Routes of administration
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)
Drug interactions
- Alkalinizing drugs (e.g., sodium bicarbonate, acetazolamide) ยท Significantly increases the renal excretion of salicylates; carbonic anhydrase inhibitors may cause systemic acidosis and increase CNS salicylate levels, leading to toxicity.
- Aminoglycosides ยท May increase the likelihood of nephrotoxicity. ยท major
- Corticosteroids ยท May increase salicylate clearance (decreasing serum levels) and significantly increase the risk of gastrointestinal ulceration and bleeding.
- Digoxin ยท Aspirin may increase plasma levels of digoxin by decreasing its clearance.
- Furosemide ยท May compete with renal excretion of aspirin, delaying its elimination and potentially causing toxicity at high doses.
- Heparin or Oral Anticoagulants ยท Increased risk of bleeding due to synergistic antiplatelet/anticoagulant effects.
- Methotrexate ยท Aspirin may displace methotrexate from plasma proteins, increasing the risk of methotrexate toxicity.
- Other NSAIDs ยท 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. ยท major
- Phenobarbital ยท May increase the rate of aspirin metabolism by inducing hepatic enzymes.
- Probenecid, Sulfinpyrazone ยท Aspirin may antagonize the uricosuric effects of these drugs.
- Spironolactone ยท Aspirin may inhibit the diuretic activity of spironolactone.
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)
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:** 1. **Decontamination:** Emesis (if within 12 hours of ingestion), activated charcoal, and an oral cathartic. Gastric lavage with 3-5% sodium bicarbonate may delay absorption. 2. **Fluid Therapy:** IV fluids (e.g., Dextrose 5% in water) to correct dehydration. 3. **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. 4. **Supportive Care:** GI protectants. Control seizures with IV diazepam. 5. **Coagulopathy Management:** Phytonadione (Vitamin K1) at 2.5 mg/kg divided q8-12h and ascorbic acid (though ascorbic acid may negate urinary alkalinization). 6. **Heroic Measures:** Peritoneal dialysis or exchange transfusions in severe cases.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.