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Kornhauser Health Sciences Library

Animal Research: Pain Categories & Procedures

This guide provides users with information on best practices for literature searches conducted for animal research.

USDA Pain Categories

The USDA classifies pain into four categories (B, C, D, and E). Protocols and annual reports must classify animals into a category and provide information on the number of animals being used and details on the types of procedures being performed. If any procedures fall into USDA’s Category D or E, causing more than momentary or slight pain or distress to the animals, the investigator must describe his/her consideration of alternatives (see the Searching the Literature tab) and the determination that alternatives are not available. 

Pain Category B

Breeding, conditioning, or holding


  • Breeding colonies of any animal species (USDA does not require listing of rats, mice, birds) that are held. Breeding colony includes parents and offspring.
  • Newly acquired animals that are being held and have not been utilized in study procedures yet.
  • Animals held under proper captive conditions or wild animals that are being observed.
  • Injections, tattooing, and blood sampling should be included in this category.

Pain Category C

No pain or distress

This category includes routine procedures which produce only momentary pain.


  • Administration of electrolytes, fluids, or oral medication
  • Blood collection from a common peripheral vein per standard veterinary practice (dog cephalic, cat jugular)
  • Standard radiography
  • parenteral injections of non-irritating substances

Pain Category D

Anesthesia, analgesia or tranquilizers are administered

Animals are expected to experience pain or distress for which appropriate anesthetic, analgesic or tranquilizing drugs will be used to alleviate.

Surgical procedures (for which pain relief was provided) such as:

  • biopsies
  • gonadectomy
  • exposure of blood vessels
  • chronic catheter implantation
  • laparotomy or laparoscopy

Blood collection by more invasive routes such as intracardiac or periorbital (in species without a true orbital sinus such as rats and guinea pigs) where pain and distress was relieved.

Administration of drugs, chemicals, toxins, or organisms that would be expected to produce pain or distress but which will be alleviated by analgesics

Prolonged restraint of an anima during which the distress will be mitigated by positive human contact and enrichment

Pain Category E

Anesthesia, analgesia or tranquilizers cannot be administered

Animals are expected to experience pain or distress for which the use of appropriate drugs would adversely affect the procedures, results, or interpretation of the research.


  • toxicity studies
  • microbial virulence testing
  • radiation sickness
  • research on stress, shock, or pain
  • surgical and postsurgical sequala from
    • invasion of body cavities
    • orthopedic procedures
    • dentistry
    • other hard or soft tissue damage that produces unrelieved pain or distress
  • chairing of nonhuman primates not conditioned to the procedure for the time period used