Phenylbutazone
Phenylbutazone (synthetic pyrazolone derivative)
Also known as: Butazolidin · Phenylbute · butadiene · fenilbutazona · phenylbutazonum
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 |
|---|---|---|---|---|---|
| General inflammation/pain | 14 mg/kg | PO | three times daily initially | — | 🌎 NA |
| General inflammation/pain | 15-22 mg/kg | PO | q12h | — | 🌎 NA |
| Analgesia | 1-5 mg/kg | PO | q8h | — | 🌎 NA |
- General inflammation/pain: Maximum of 800 mg/day regardless of weight; titrate dose to lowest effective dose. Rarely used today.
Horses
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| To reduce pain or pyrexia associated with pleuropneumonia | 2.2-4.4 mg/kg | PO or IV | q12h | first week of treatment or longer | 🌎 NA |
| General musculoskeletal inflammation | 1-2 grams per 454 kg (1000 lb.) horse IV, or 2-4 grams per 454 kg (1000 lb.) horse PO | IV or PO | daily | Limit IV administration to no more than 5 successive days | 🌎 NA |
| Adjunctive treatment of colic (to reduce endotoxic effects) | 2.2 mg/kg | PO or IV | twice daily | — | 🌎 NA |
| Adjunctive treatment of laminitis | Initial dose of 4 grams, immediately decreased to 1 to 1.5 grams | PO or IV | twice daily | — | 🌎 NA |
| Osteoarthritis | 2.2 mg/kg | PO | twice daily | — | 🌎 NA |
- General musculoskeletal inflammation: Do not exceed 4 grams/day. Use high end of dosage range initially, then titrate to lowest effective dose. Injection must be made slowly and with care.
- Adjunctive treatment of laminitis: Dose for an average adult-sized horse. Lower dose is used to keep the horse comfortable, but not relieve pain to the extent the horse moves around excessively.
- Osteoarthritis: Minimal dose to control pain.
Cattle
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| General inflammation/pain | 4-8 mg/kg PO or 2-5 mg/kg IV | PO or IV | — | — | 🌎 NA |
| General inflammation/pain | 10-20 mg/kg PO, then 2.5-5 mg/kg q24h or 10 mg/kg every 48 hours PO | PO | q24h or q48h | — | 🌎 NA |
- General inflammation/pain: Extralabel use prohibited in female dairy cattle 20 months of age or older.
- General inflammation/pain: Extralabel use prohibited in female dairy cattle 20 months of age or older.
Swine
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| General inflammation/pain | 4 mg/kg | IV or PO | q24h | — | 🌎 NA |
| General inflammation/pain | 4-8 mg/kg PO or 2-5 mg/kg IV | PO or IV | — | — | 🌎 NA |
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Phenylbutazone (commonly known as "bute") is a classic, non-selective non-steroidal anti-inflammatory drug (NSAID) of the pyrazolone class.
Clinical Pearl: While historically used across many species for its potent analgesic, anti-inflammatory, and antipyretic properties, its use in modern veterinary medicine is almost exclusively restricted to equine practice for the management of musculoskeletal pain (e.g., laminitis, osteoarthritis).
In small animals (dogs and cats), it has largely been replaced by safer, COX-2 selective NSAIDs (like carprofen or meloxicam) due to a high risk of severe adverse effects, including blood dyscrasias, hepatotoxicity, and severe gastrointestinal ulceration. It is strictly regulated in food-producing animals due to human safety concerns regarding bone marrow toxicity (aplastic anemia).
Mechanism of action
Phenylbutazone acts primarily by inhibiting the cyclooxygenase (COX) enzymes (both COX-1 and COX-2), which prevents the conversion of arachidonic acid → prostaglandins and thromboxanes.
- ↓ Prostaglandins → Reduced inflammation, analgesia, and antipyresis.
- ↓ Thromboxane A2 → Decreased platelet aggregation.
Pharmacological Note: Phenylbutazone is metabolized in the liver to oxyphenbutazone, an active metabolite that contributes significantly to its prolonged efficacy (often >24 hours) despite a relatively short plasma half-life. The irreversible binding of phenylbutazone to cyclooxygenase also extends its duration of action. It also possesses mild uricosuric properties.
Safety & warnings
Contraindications
- Known hypersensitivity to phenylbutazone or other NSAIDs
- History of or preexisting hematologic or bone marrow abnormalities
- Preexisting gastrointestinal ulcers
- Food-producing animals (especially female dairy cattle 20 months of age or older)
- Lactating dairy cattle
- Pre-existing gastrointestinal ulceration
- Renal impairment
- Hepatic impairment
- Hematological disorders
- Use in food-producing animals (banned in many jurisdictions)
Adverse effects
- Gastrointestinal and oral erosions/ulcers
- Hypoalbuminemia (especially in foals/ponies)
- Renal papillary necrosis and azotemia
- Sodium and water retention (edema)
- Blood dyscrasias (agranulocytosis, aplastic anemia)
- Hepatotoxicity
- Severe tissue necrosis and sloughing (if injected IM or SC)
- CNS stimulation and seizures (if injected intracarotid)
- Gastrointestinal ulceration and bleeding
- Bone marrow suppression (aplastic anemia, leukopenia)
Precautions
Use with extreme caution in foals and ponies due to increased incidence of hypoproteinemia and GI ulceration. Avoid in horses with known or suspected Equine Gastric Ulcer Syndrome (EGUS). Use cautiously in animals with preexisting renal disease or congestive heart failure (CHF) due to potential for sodium/water retention and decreased renal blood flow. May mask clinical signs of lameness in horses. WARNING: Do not administer injectable preparations IM or SC as it is highly irritating and causes necrosis/sloughing. Intracarotid injections may cause CNS stimulation and seizures.
Drug interactions
Phenylbutazone may antagonize the increased renal blood flow effects caused by furosemide.
Concurrent administration may increase the chances of hepatotoxicity developing.
Concurrent use increases the potential for adverse GI and renal reactions. (Note: 'Stacking' with ketoprofen shows synergistic toxicity).
May increase the risk of hematologic and/or renal adverse reactions.
Phenylbutazone may increase the plasma half-life of penicillin G.
Phenylbutazone could potentially displace sulfonamides from plasma proteins, increasing the risk for adverse effects.
Phenylbutazone could potentially displace warfarin from plasma proteins, increasing the risk for bleeding.
Increased risk of gastrointestinal ulceration
Increased risk of gastrointestinal ulceration and renal toxicity
Displacement from plasma proteins leading to increased free drug concentrations
Monitoring
- Analgesic, anti-inflammatory, and antipyretic efficacy
- Complete blood counts (CBC) with chronic therapy (weekly early in therapy, biweekly later)
- Urinalysis and renal function parameters (serum creatinine/BUN) with chronic therapy
- Plasma protein determinations (especially in ponies, foals, and debilitated animals)
- Complete Blood Count (CBC)
- Renal function (BUN, Creatinine)
- Liver enzymes
- Clinical signs of GI bleeding (melena)
Pharmacokinetics
Half-life
Absorption
Absorbed from both the stomach and small intestine following oral administration.
Distribution
Distributed throughout the body with highest levels attained in the liver, heart, lungs, kidneys, and blood. Plasma protein binding in horses exceeds 99%. Crosses the placenta and is excreted into milk.
Metabolism
Nearly completely metabolized in the liver, primarily to oxyphenbutazone (active) and gamma-hydroxyphenylbutazone.
Elimination
Excreted primarily in the urine. More rapidly excreted into alkaline than acidic urine. Oxyphenbutazone can be detected in horse urine up to 48 hours after a single dose.
Overdose
Manifestations of acute overdosage include prompt respiratory or metabolic acidosis with compensatory hyperventilation, seizures, coma, and acute hypotensive crisis.
- Systemic Effects: Clinical signs of renal failure (oliguric, proteinuria, hematuria), liver injury (hepatomegaly, jaundice), bone marrow depression, and severe GI ulceration/perforation may develop.
- Dogs: Common signs include ataxia, seizures, tachycardia, trembling, tremors, vomiting, and tachypnea. Oral LD50 is 332 mg/kg.
- Horses: Common signs include colic, anorexia, and ataxia.
Treatment: Standard overdose procedures should be followed (e.g., empty gut following oral ingestion). Institute supportive treatment as necessary. Intravenous diazepam can be used to help control seizures. Monitor fluid therapy carefully, as phenylbutazone may cause fluid retention.
Available products
Formulations
- Tablets
- Oral powder
- Oral paste
- Injection
- 100 mg tablet
- 200 mg tablet
Veterinary
- Phenylbutazone Tablets: 100 mg, 200 mg, 1 gram
- Phenylbutazone Oral Powder: 1 gram in 10 grams of powder
- Phenylbutazone Paste Oral Syringes: 6 grams or 12 grams/syringe
- Phenylbutazone Injection: 200 mg/mL in 100 mL vials
- 100 mg tablets (Currently unavailable)
- 200 mg tablets (Currently unavailable)
Regulatory status
POM. Licensed forms for small animals are currently unavailable. Prohibited in food-producing animals.
POM-V. Licensed forms for small animals are currently unavailable. Prohibited in food-producing animals.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Oral products should be stored in tight, child-resistant containers if possible. The injectable product should be stored in a cool place (46-56° F) or kept refrigerated.
Client information
For Horse Owners:
- "Bute" is highly effective for managing bone and joint pain. Always administer exactly as prescribed by your veterinarian.
-
CRITICAL WARNING: Never administer the liquid injectable form into the muscle (IM) or under the skin (SC). It must be given strictly into the vein (IV) by a professional. Incorrect injection causes severe tissue damage and sloughing.
- Watch closely for signs of stomach ulcers or kidney issues: loss of appetite, diarrhea, signs of colic, or swelling in the legs/belly. Stop the medication and contact your vet if these occur.
For Dog Owners:
- This medication is rarely used in dogs today due to the availability of safer alternatives. If prescribed, monitor your dog very closely for vomiting, dark/tarry stools, or lethargy, and contact your vet immediately if these occur.
Food Animal Restrictions:
- Strictly banned by the FDA for use in female dairy cattle 20 months of age or older.
- If used extralabel in beef cattle, strict meat withdrawal times apply (e.g., 1 dose = 30 days, 2 doses = 35 days). Consult your veterinarian.
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.