Tolfenamic Acid
Tolfenamic acid is a nonsteroidal anti-inflammatory drug (NSAID) belonging to the **anthranilic acid (fenamate)** class, chemically related to meclofenamic acid. It is utilized primarily for its analgesic, anti-inflammatory, and antipyretic properties. **Key Clinical Features:** * **Dual Action:** Unlike many traditional NSAIDs, fenamates like tolfenamic acid not only inhibit prostaglandin synthesis but also directly block prostaglandin receptors, potentially offering enhanced analgesic efficacy. * **Pulse Dosing:** For chronic conditions in dogs, it is uniquely prescribed on a "pulse" schedule (e.g., 3-5 consecutive days per week) to minimize gastrointestinal and renal toxicity while maintaining efficacy. * **Platelet Effects:** It possesses significant anti-thromboxane activity, which impairs platelet aggregation. Therefore, it is **not recommended for pre-surgical use**. * **Availability:** While widely approved in Canada, Europe, and Australia for small and large animals, it is currently not available in the USA. > **Clinical Pearl:** Because enterohepatic recirculation is increased when given with food, administering tolfenamic acid with a meal can increase its bioavailability, though it may also increase absorption variability compared to a fasted state.
Mechanism: Tolfenamic acid exerts its effects through multiple pathways in the inflammatory cascade: * **Cyclooxygenase (COX) Inhibition:** It is a potent inhibitor of the **COX** enzyme, which prevents the conversion of arachidonic acid โ **prostaglandins (PGs)** and **thromboxanes (TXA2)**. This reduces inflammation, pain, and fever. * **Direct Receptor Antagonism:** Uniquely among NSAID classes, fenamates directly block the binding of synthesized prostaglandins to their respective tissue receptors (e.g., EP receptors), providing a secondary mechanism of action against pain and inflammation. * **Anti-thromboxane Activity:** By inhibiting TXA2 synthesis, it significantly impairs platelet aggregation and normal clotting function.
Dosing by species
- Acute pain ยท 4 mg/kg ยท SC, IM or PO ยท once daily ยท 3-5 days (injectable suggested for first dose only)
- Pain/inflammation ยท 4 mg/kg ยท PO ยท once daily ยท 3-5 days or as recommended by the veterinarian
- Inflammation, pain, and fever ยท 4 mg/kg ยท SC ยท once, may be repeated once after 24 hours ยท 1-2 days ยท Do NOT give IM to cats. Treatment starts with a single injection on day 1.
- Inflammation, pain, and fever ยท 4 mg/kg ยท PO ยท sid ยท 3 days ยท Treatment starts with a single injection on day 1, followed by 3 days PO. Repeated dosing on a weekly basis is NOT recommended in cats.
- Metritis-mastitis-agalactia (MMA) ยท 2 mg/kg ยท IM ยท once ยท Once ยท Meat withdrawal = 6 days.
- Pneumonia ยท 2 mg/kg ยท IM ยท Treatment may be repeated once only after 48 hours ยท Up to 2 doses ยท Inject high in the neck. Meat withdrawal = 10 days.
- Mastitis ยท 4 mg/kg ยท IV ยท single injection ยท Once ยท Meat withdrawal = 4 days; Milk withdrawal = 12 hours (1 milking).
- Acute pain ยท 4 mg/kg ยท SC, IM or PO ยท once daily ยท 3-5 days (injectable suggested for first dose only)
Routes of administration
Contraindications
- Hypersensitivity to tolfenamic acid or other fenamates (e.g., meclofenamic acid)
- Active gastrointestinal bleeding or ulceration
- Pre-surgical administration (due to anti-thromboxane/platelet effects)
- Dehydrated, hypovolemic, or hypotensive patients
- Patients with gastrointestinal disease
- Patients with blood clotting problems
- Pregnant animals
- Animals under 6 weeks of age
- Intramuscular (IM) administration in cats
Adverse effects
- Vomiting
- Diarrhea
- Gastrointestinal ulceration (at high doses)
- Platelet dysfunction/prolonged bleeding time
- Potential nephrotoxicity (especially in dehydrated patients)
- Gastrointestinal signs (vomiting, diarrhea, anorexia)
- Gastrointestinal ulceration and bleeding
- Renal toxicity (especially during hypotension)
- Potential precipitation of cardiac failure (rare/unknown risk in animals)
Drug interactions
- Aspirin ยท May increase the risk of gastrointestinal toxicity (e.g., ulceration, bleeding, vomiting, diarrhea).
- Corticosteroids ยท Concomitant therapy may increase the occurrence of gastric ulceration; avoid concurrent use.
- Digoxin ยท NSAIDs may increase serum levels of digoxin.
- Fluconazole ยท May increase plasma levels of NSAIDs; potentially affects tolfenamic acid levels in dogs.
- Furosemide ยท NSAIDs may reduce the saluretic and diuretic effects of furosemide.
- Methotrexate ยท Serious toxicity has occurred with concomitant NSAID use; use together with extreme caution.
- Nephrotoxic Drugs (e.g., aminoglycosides, amphotericin B) ยท May enhance the risk of nephrotoxicity.
- Other NSAIDs ยท May increase the risk of gastrointestinal toxicity (e.g., ulceration, bleeding, vomiting, diarrhea). ยท major
- Warfarin ยท Tolfenamic acid is 98-99% protein-bound; may displace highly protein-bound drugs like warfarin, increasing bleeding risk. Monitor closely.
- Glucocorticoids ยท Increased risk of gastrointestinal ulceration and bleeding. Do not administer concurrently or within 24 hours. ยท major
- Aminoglycosides ยท Increased risk of nephrotoxicity. ยท major
Monitoring
- Clinical efficacy (pain scores, resolution of inflammation)
- Adverse effects (vomiting, diarrhea, anorexia)
- Renal function (BUN, Creatinine, SDMA, USG) with prolonged use
- Signs of GI bleeding (melena, pale mucous membranes)
- Clinical signs of gastrointestinal toxicity (vomiting, diarrhea, melena, anorexia)
- Renal parameters (BUN, creatinine, USG), especially in perioperative or hypotensive patients
- Liver enzymes in patients receiving prolonged therapy
Overdose
Acute toxicity data is limited. Experimental studies in dogs and cats did not demonstrate significant renal or GI toxicity until doses exceeded 10 times the labeled dose. **Treatment Protocol:** * **Decontamination:** Empty the gut following recent oral ingestion (emesis induction, activated charcoal). * **Supportive Care:** Institute IV fluid therapy to maintain hydration and support renal perfusion. * **Seizure Control:** Use IV **diazepam** if seizures occur. * **Monitoring:** Closely monitor for signs of gastrointestinal bleeding (melena, hematemesis). Monitor electrolyte and fluid balance carefully, and manage any acute renal failure using established veterinary guidelines.
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.