title: Factors affecting drug withholding time estimates in horses.
abstract: Although all the factors discussed in this article may have an effect on drug withholding time estimates, the factors that have the potential for the greatest effect or that have been found to cause positive tests in the past are 1. Dosage: Increasing the drug dosage will require a longer withholding time. 2. Dosing interval: Narrowing the dosing interval will require a longer withholding time. 3. Administration route: In general, oral administration results in lower peak plasma concentrations but may result in longer excretion in the urine and therefore longer withholding time. 4. Drug interaction: The co-administration of drugs may result in interference with the clearance of either drug requiring longer withholding times. It should be noted that environmental chemicals, natural substances in feeds, and other substances have the potential to alter the rate of elimination of a drug. 5. Exercise: Exercise may have a significant effect on the excretion of a drug or its metabolites in the urine. The magnitude of this effect is primarily dependent on the nature of the drug and the mechanisms involved in the renal excretion of the drug. These effects may be important if experimental studies in resting horses are used to estimate withholding times. 6. Analytic method: A change in a screening test for a therapeutic medication by the laboratory may dramatically increase the withholding time for that drug. Veterinarians need to work closely with racing commissions and horse show associations to encourage them to provide advanced notice of such changes and to provide revised withholding time estimates. The attending veterinarian should explain the caveats of withholding time estimates to the trainer or owner to avoid misunderstandings in the event of a positive test. However, the decision to enter the horse in competition after drug treatment is ultimately the responsibility of the trainer and owner.
authors.affiliation: Department of Veterinary Physiology and Pharmacology, Ohio State University College of Veterinary Medicine, Columbus.
authors.forename: D F
authors.initials: DF
authors.lastname: Gerken
authors.forename: R A
authors.initials: RA
authors.lastname: Sams
mesh_terms: D000818:Animals; D004305:Dose-Response Relationship, Drug; D004333:Drug Administration Routes; D004334:Drug Administration Schedule; D004347:Drug Interactions; D006736:Horses / Q000378:metabolism*; D004364:Pharmaceutical Preparations / Q000032:analysis; D010599:Pharmacokinetics; D010805:Physical Conditioning, Animal; D013997:Time Factors
chemical_list: D004364:Pharmaceutical Preparations