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  • Phenylbutazone.

Class of drug

  • Non-steroidal anti-inflammatory.


Chemical name

  • 4-butyl-1,2-diphenyl-pyrazolidine-3,5-dione.

Molecular formula

  • C19H20N2O2.

Molecular weight

  • 308.374 g/mol.

Physical properties

  • Lipophilic.

Storage requirements

  • Room temperature.


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  • Inflammation, pain and pyrexia.

Some authorities, including the Jockey Club, regard phenylbutazone as a 'prohibited substance' under the rules of competition. Therefore, use of this product in a competition horse should be in accordance with the recommendations/advice of the relevant competition authorities. The clinical effect of phenylbutazone can be evident for at least three days following cessation of administration. This should be bourne in mind when examining horses for soundness.


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with other drugs

Drugs affecting phenylbutazone
  • Barbiturates - decreased metabolism.
  • Cardiac glycosides - increased metabolism and decreased effect.
  • Chloramphenicol - decreased metabolism.
  • Colestyramine - decreased enteral absorption.
  • Corticosteroids, other NSAIDs and sulfonamides - displacement from plasma proteins and increased effect.

Drugs affected by phenylbutazone

  • Phenylbutazone is extensively bound to plasma proteins; it may displace other drugs that are highly protein-bound, or it may itself be displaced to produce an increase in non-bound pharmacologically active concentrations which can lead to toxic effects.
  • Concurrent therapy with other therapeutic agents should be undertaken with caution due to the risk of metabolic interactions. There is evidence to indicate that the pharmacokinetics of penicillin and gentamicin products may be affected by concurrent administration of products containing phenylbutazone with a possible reduction of therapeutic efficacy, since tissue penetration may be reduced.
  • The distribution of other drugs given concurrently may also be affected.
  • Phenylbutazone induces hepatic microsomal enzyme activity.
  • Methotrexate Methotrexate - decreased excretion.
  • Penicillins Penicillin G - altered half-life and tissue penetration.
  • Phenytoin Phenytoin - decreased metabolism.
  • Sulfonylureas - increased hypoglycemic effect.
  • Levothyroxine Levothyroxine - falsely low plasma-total levothyroxine.
  • Warfarin Warfarin - increased anticoagulant effect.

Adverse Reactions

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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Authie E C et al (2010) Effect of an endurance-like exercise on the disposition and detection time of phenylbutazone and dexamethasone in the horse: Application to medication control. Equine Vet J 42 (3), 240-247 PubMed.
  • Sabate D, Homedes J, Salichs M, Sust M & Monreal L (2009) Multicentre, controlled, randomised and blinded field study comparing efficacy of suxibuzone and phenylbutazone in lame horses. Equine Vet J 41 (7), 700-705 PubMed.
  • Conwell R & Stratford C (2008) Right dorsal colitis. Vet Rec 13 (8), 7-10.

Other sources of information

  • Derived from The Veterinary Formulary (1998) 4th edn. Ed: Bishop Y. British Veterinary Association and Royal Pharmaceutical Society.

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Abdomen: surgical approaches

Accessory ligament DDFT: desmotomy

Accessory ligament DDFT: forelimb - desmitis

Accessory ligament SDFT: desmotomy

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Anesthesia: analgesia - overview

Anesthesia: peri-operative complications

Annular ligament: transection - palmar / plantar


Bladder: urinary tract infection

Blood: platelet evaluation

Bone: physitis



Carotid artery: intracarotid injection

Carpus: chip removal

Carpus: fracture - chip

Carpus: sheath tenosynovitis

Cecum: typhlectomy

Cecum: typhlotomy


Common digital extensor tendon: rupture

Cranial nuchal bursa bursoscopy

Deep digital flexor tendon: tendinitis

Deep digital flexor tendon: tenotomy

Dermatology: intradermal test

Digital sheath: tenosynovitis

DIP joint: flexural deformity

Distal phalanx: fracture repair

Distal phalanx: fractures

Distal phalanx: pedal osteitis - septic

Elbow: osteoarthritis

Endocrine: hypothyroidism

Endocrine: T3 assay

Endocrine: T4 assay

Endotoxemia: overview

Epiglottis: entrapment - repair

Equine viral encephalitides

Esophagus: obstruction

Ethmoid: hematoma

Extensor carpi radialis muscle: rupture

Eye: dermoid excision

Eye: glaucoma

Eye: intraocular prosthesis

Eyeball: enucleation - transconjunctival

Flexor tendon: trauma

Foal: resuscitation and care

Foot: hoof wall - crack repair

Foot: keratoma - resection

Foot: laminitis

Foot: sole bruising

Foot: subsolar abscess/infection

Gastrointestinal: ileus

Grass sickness

Hard palate: palatitis


Joint: septic arthritis - adult

Keratitis: bacterial

Keratitis: traumatic - ulcerative

Larynx: arytenoidectomy

Larynx: ventriculocordectomy

Laser surgery: transendoscopic



Lung: pleuropneumonia - bacterial (pleuritis)

Lung: pneumonia - neonatal bacterial


Male: ejaculatory dysfunction

Mammary gland: mastitis

MC / MT 2 and 4: fracture repair

MC / MT 2 and 4: partial removal

MC / MT 3: condylar fracture repair

MC / MT 3: diaphysis fracture

MC / MT 3: dorsal metacarpal disease

MC / MT 4: removal

MC/MT 2 and 4: fracture

MC/MT 2 and 4: periostitis (splints)

MCP / MTP joint: chronic proliferative synovitis

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Muscle: myopathy - fibrotic

Muscle: myopathy - sporadic exertional rhabdomyolysis

Musculoskeletal: back pain

Musculoskeletal: fracture - first aid

Musculoskeletal: osteoarthritis (joint disease)

Navicular bone: suspensory desmotomy

Navicular bone: syndrome

Navicular bursa: puncture


Orbit: trauma

Palmar carpal retinaculum: section

Palmar digital nerve: neurectomy

Patella: fracture

Patella: fragmentation

Pelvis: fracture

Pelvis: trauma - sacroiliac

Penis: phallectomy - Scott's technique

Penis: phallectomy - Vinscot technique

Penis: phallectomy - William technique

Perineum: trauma - repair

Peripheral nerve: trauma

Pharynx: ventral pharyngotomy


Photoactivated vasculitis

PIP joint: arthrodesis

PIP joint: luxation and subluxation

PIP joint: osteoarthritis

Placenta: retained

Prepuce: resection

Proximal phalanx: fracture

Proximal sesamoid: fracture

Proximal sesamoid: fracture - arthroscopic removal

Purpura hemorrhagica

Rectum: meconium impaction

Reproduction: gamete intrafallopian transfer

Reproduction: oocyte transfer

Respiratory: anesthesia

Respiratory: meconium aspiration

Respiratory: neonatal respiratory distress syndrome

Respiratory: smoke inhalation

Sacrum: fracture

Sciatic nerve: paralysis

SDFT: luxation

SDFT: tendinitis

Spine: cervical vertebral malformation (CVM)

Spine: fracture

Sternum: bone graft collection

Stifle: femoropatellar osteochondrosis

Stifle: femorotibial subchondral bone cyst

Stifle: trauma - collateral ligament

Stifle: trauma - cruciate ligament

Stifle: trauma - meniscal

Stomach: gastric ulceration

Straight sesamoidean ligament: desmitis

Suprascapular nerve: paralysis

Suspensory ligament: desmitis - midbody

Suspensory ligament: desmitis - proximal

Tarsus: arthrodesis

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Tarsus: fracture

Tarsus: osteoarthritis (bone spavin)

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Testis: castration - post-operative complications

Testis: orchitis

Therapeutics: anti-inflammatory drugs

Therapeutics: gastrointestinal system

Therapeutics: skin

Tibia: bone graft collection

Tibia: fracture - tibial crest

Toxicity: anticoagulant rodenticide

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Tuber coxae: bone graft collection

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Ulnaris lateralis: tenotomy - flexural deformity


Uterus: caesarean section

Uveitis: traumatic


Vesicular stomatitis

Warfarin sodium

Wound: post-operative care

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