Equis ISSN 2398-2977

Phenylbutazone

Contributor(s): Nicola Menzies-Gow, Vetstream Ltd

Introduction

Name

  • Phenylbutazone.

Class of drug

  • Non-steroidal anti-inflammatory.

Description

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.

Uses

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Indications

  • 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.

Administration

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Pharmocokinetics

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Precautions

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Interactions

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

Publications

Refereed papers

  • Recent references fromPubMed andVetMedResource.
  • Authie E Cet 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 J42(3), 240-247PubMed.
  • 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 J41(7), 700-705PubMed.
  • Conwell R & Stratford C (2008)Right dorsal colitis.Vet Rec13(8), 7-10.

Other sources of information

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

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