VPT 421: Veterinary Toxicology (2+0)

WITHDRAWAL TIME

Definition

• The withdrawal time is the time from the cessation of treatment to the time it takes for the residues of the drug to deplete below the safe concentration. Withdrawal times for the FDA approved drugs for use in food animals are only valid for the specified species, dose, route and frequency of administration. They are also specific to the manufacturer’s product and formulation: thus, a drug substance may have different withdrawal times when present in the differently formulated drug products. This term is often used more broadly to describe the time needed after drug administration to any food animal where drug residue may be found in marketed meats, eggs, organs, or other edible products.
• The withdrawal time is based on the time for the average human to ingest 1/100 or 1/2000 of the NOEL.
• The withdrawal period is the time required for the residue of toxicological significance to reach a safe concentration as defined by the tolerance.

Safe concentration = (ADI x 60 kg )/ 500 g x Food factor

• Withdrawal periods reflect the amount of time necessary for an animal to metabolize an administered product and the amount of time necessary for the product concentration level in the tissues to decrease to a safe, acceptable level.

Withdrawal periods for meat and milk

• Every federally approved drug or animal health product has a withdrawal period printed on the product label or package insert.
• Products carry meat withdrawal periods ranging from 0 to 60 days.
• Examples for meat range from
  • no withdrawal period with ceftiofur
  • 4-15 days with different penicillin products
  • 28 days with Pirlimycin.
• Animals treated with a product that has a withdrawal period of 45 days should be withheld from sale or slaughter for at least 45 days.
• Withdrawal times are not the same for all drugs.
• Examples for milk include:
  • Pirlimycin - 36 hours
  • Cloxacillin - 48 hours,
  • Amoxicillin - 60 hours,
  • Penicillin - 72 hours
  • Cephapirin - 96 hours.
• Milk produced during that period must be disposed off.
• Withdrawal periods may be extended when combinations of drugs are used or when drugs are used in an extra-label manner.