What are the veterinary regulations governing prescription paste formulations like phenylbutazone and ivermectin?

1. Approval pathway and labeling: what converts a paste into a regulated prescription drug

Paste formulations become regulated veterinary prescription drugs when a New Animal Drug Application (NADA) or Abbreviated NADA is approved by FDA’s Center for Veterinary Medicine, which then amends 21 CFR to include the product’s formulation, concentration, approved species, indications, and dosing instructions—as shown for phenylbutazone paste and multiple ivermectin products approved via Federal Register notices and FDA summaries ^{[1] [2] [3] [6]}.

2. Prescription status and veterinary oversight: the legal requirement

Federal law and FDA policy require that many of these products be dispensed only “by or on the order of a licensed veterinarian,” meaning label status is prescription-only and use requires veterinary oversight; FDA explains that when directions for safe use cannot be written for lay users, a prescription/VCPR requirement applies ^{[4] [5] [2]}.

3. Species-specific approvals and labeled indications: no blanket, cross‑species use

Each approved paste is labeled for particular species and clinical indications—ivermectin pastes are often approved specifically for horses with enumerated parasite targets, and phenylbutazone paste is approved for relief of inflammatory musculoskeletal conditions in horses—so the law ties use to those labeled species and conditions ^{[7] [1] [6]}.

4. Extra‑label use: allowed but circumscribed, with hard prohibitions for food animals

Veterinarians may engage in extra‑label use under the Animal Medicinal Drug Use Clarification Act framework, but FDA and the CFR place limits: extra‑label use is prohibited in certain food‑producing species for specified purposes, and unapproved doses, routes, frequencies, or major food‑producing species/classes can be disallowed—labels and 21 CFR sections reflect these constraints and explicit withdrawal/withholding times where applicable ^{[4] [5]}.

5. Safety communications and public-health guardrails: FDA warnings against off‑label human use

FDA has repeatedly warned that ivermectin intended for animals should not be used in humans and emphasized that animal ivermectin must be used only for approved animal indications or as prescribed by a veterinarian consistent with extra‑label drug use rules—this reflects both safety and regulatory concerns when animal paste products are diverted for unintended uses ^{[8] [9]}.

6. Drug interactions, professional guidance, and label limitations

Labels and professional guidance note important drug interactions and contraindications—for example, phenylbutazone and other NSAIDs should not be combined in horses—information that is part of approved labeling and professional-policy guidance which underpins why veterinary oversight is required ^{[10] [1]}.

7. What the record does not show: compounding, state practice variation, and enforcement details

The provided sources document federal approvals, labeling language, prescription status, and extra‑label prohibitions, but they do not supply a comprehensive account of state pharmacy practice variations, FDA enforcement actions for specific paste misuse, or the full scope of compounding rules as applied to these pastes; those are outside the supplied reporting and therefore cannot be asserted here ^{[2] [4] [5]}.

8. Practical implications and tensions: access vs. safety, and public misuses

The regulatory regime aims to balance animal‑health access to effective antiparasitics and analgesics with public‑health protections—by limiting lay use and by defining species/dose restrictions—yet real‑world tensions arise when demand spikes or misinformation encourages human use of animal pastes, prompting FDA stakeholder letters and FAQs warning against such practices ^{[8] [9] [5]}.