Not every medication a veterinary patient needs exists in an FDA-approved formulation. Some animals need a different strength, a different flavor, a different route of administration, or a drug that has no approved animal version at all. That is where compounding comes in — and where FDA Guidance for Industry #256 (GFI #256) draws the line between acceptable pharmacy practice and enforcement risk.

This article explains what GFI #256 changed, how the rules differ for patient-specific prescriptions versus office stock, what documentation your practice must maintain, and how to evaluate whether a compounding pharmacy is operating within federal guidelines.

What GFI #256 Covers

Published in final form in April 2022, GFI #256 is the FDA Center for Veterinary Medicine's current guidance on compounding animal drugs from bulk drug substances. It does not make compounding from bulk drug substances legal — those drugs are still considered unapproved "new animal drugs" under the Federal Food, Drug, and Cosmetic Act. Instead, the guidance describes the circumstances under which the FDA does not generally intend to take enforcement action.

The guidance applies to:

• Nonfood-producing animals — dogs, cats, horses not intended for food, and other companion animals where bulk compounding is most common.
• Limited food-animal uses — antidotes for toxicoses in food-producing animals, and sedatives or anesthetics for free-ranging wildlife, when no other medically appropriate option exists.
• State-licensed pharmacies, federal facilities, and veterinarians compounding within their license.

Compounding that starts from an FDA-approved drug (human or animal) as the active ingredient source, rather than from a bulk substance, is governed by separate extralabel-use regulations under AMDUCA (21 CFR Part 530). GFI #256 specifically addresses compounding from bulk drug substances — the raw active pharmaceutical ingredient.

Patient-Specific Prescriptions Versus Office Stock

GFI #256 creates two distinct pathways, each with different documentation and enforcement expectations.

Patient-specific prescriptions

When a veterinarian prescribes a compounded drug for an identified patient, the compounding pharmacy fills that prescription and ships it directly. The FDA considers this the lower-risk pathway because the veterinarian has made an individualized clinical decision.

Key requirements:

• A valid veterinarian-client-patient relationship (VCPR) must exist.
• There must be no medically appropriate FDA-approved or indexed drug available, or no lawful extralabel use of an approved drug, that can treat the animal's condition.
• The compounding must be performed by or under the direct supervision of a licensed pharmacist in a state-licensed pharmacy, or by a veterinarian, or in a federal facility.
• All state laws and regulations must be followed. When state and federal requirements conflict, the more stringent applies.
• The compounding pharmacy must report adverse events and product defects to the FDA within 15 business days using Form FDA 1932a.

Office stock

Office stock is compounded medication kept on hand in a veterinary clinic for urgent or immediate use, rather than filled for a specific patient in advance. The FDA has expressed particular concern about office stock because it is produced in larger quantities and can affect many animals if compounded improperly.

Under GFI #256, a bulk drug substance may be used for office stock only if:

• It appears on the FDA's List of Bulk Drug Substances for Compounding Office Stock Drugs for Use in Nonfood-Producing Animals, or
• It has been nominated for the list and is currently under review by the FDA. The FDA has stated it does not intend to take action against substances under active review.

If a substance has been reviewed and not listed, it may not be used for office stock. It may still be compounded for individual patient prescriptions with appropriate medical rationale documentation.

The FDA maintains three dynamic lists that are updated on a rolling basis:

Documentation Your Practice Must Maintain

GFI #256 does not require veterinarians to submit documentation to the FDA proactively. However, the guidance states that if the FDA requests records during an investigation, veterinarians should be able to produce:

1. The medical rationale for why no FDA-approved or indexed drug was medically appropriate. The FDA has specifically noted that broad or imprecise dropdown-menu justifications (for example, "patient allergic to FDA-approved products" without naming the specific ingredient) are not sufficient on their own.

2. The prescription or order identifying the patient, the drug, the strength, the dosage form, and the route of administration.

3. Adverse event reports — if your practice becomes aware of an adverse reaction or product defect related to a compounded drug, you should report it and retain the report. The FDA Form 1932a is used for animal drug adverse event reporting.

4. Labeling — compounded drugs must be labeled with the information specified in the guidance, including the active ingredients, strength, dosage form, beyond-use date, storage conditions, the name of the prescribing veterinarian, and the name and address of the compounding pharmacy.

How to Evaluate a Compounding Pharmacy

Not all compounding pharmacies are equal. Practices that prescribe compounded medications should evaluate their pharmacy partners against these criteria:

Licensing and accreditation

• Is the pharmacy licensed in your state? Verify through your state board of pharmacy.
• Does the pharmacy hold PCAB (Pharmacy Compounding Accreditation Board) accreditation or NABP (National Association of Boards of Pharmacy) verification?
• Does it clearly state whether it operates under Section 503A (patient-specific) or 503B (outsourcing facility, which permits larger-batch production under cGMP)?

GFI #256 compliance

• Does the pharmacy ask for medical rationale when prescribing from bulk drug substances that are not on the FDA office stock list?
• Does it refuse to compound copies of commercially available drugs without documented justification?
• Can it provide Certificates of Analysis (CoAs) for its compounded products, showing identity, potency, and sterility testing where applicable?
• Does it report adverse events to the FDA?

Quality and safety practices

• Does the pharmacy test its finished products for potency, sterility, and endotoxins (for injectables)?
• Are beyond-use dates established through stability testing rather than arbitrary assignment?
• Does the pharmacy specialize in veterinary compounding, or does it primarily serve human patients? Veterinary-focused pharmacies are more likely to understand species-specific formulation requirements, excipient safety (for example, avoiding xylitol or certain preservatives in animal formulations), and FDA animal drug regulations.

Red flags

Avoid pharmacies that:

• Market compounded copies of commercially available FDA-approved drugs without requiring individualized medical justification.
• Offer bulk drug substances for office stock that are not on the FDA's listed or under-review lists.
• Cannot provide documentation of quality testing.
• Use electronic dropdown menus as the sole documentation for medical rationale without supporting clinical detail.
• Advertise compounded alternatives to FDA-approved drugs as cost-saving measures without disclosing that compounded drugs are not FDA-approved and have not been evaluated for safety and effectiveness through the approval process.

Common Compliance Pitfalls

Treating office stock like regular inventory ordering. Office stock compounded from bulk drug substances is limited to the FDA's listed substances. Practices that order bulk-compounded office stock outside the listed or under-review categories are exposed to enforcement action, even if a compounding pharmacy is willing to fill the order.

Assuming the pharmacy handles all compliance. GFI #256 places responsibilities on both the compounding pharmacy and the prescribing veterinarian. The veterinarian must have a valid VCPR, must determine that no approved drug is medically appropriate, and must be prepared to document the clinical rationale if asked.

Using compounding to bypass the approval process. The FDA's first enforcement priority is drugs compounded from bulk that are essentially copies of FDA-approved products. If an approved drug exists at the right strength and dosage form and can treat the condition, the compounded version is a copy. Using a compounded version to save cost, avoid a prescription requirement, or obtain a slightly different formulation without clinical justification does not meet the GFI #256 standard.

Ignoring state law. Some states have their own compounding regulations that are more restrictive than GFI #256. In any conflict between state and federal rules, the more stringent requirement applies. Check your state veterinary practice act and board of pharmacy regulations.

Not reporting adverse events. Both the compounding pharmacy and the veterinarian should report adverse events related to compounded drugs to the FDA. The reporting form is FDA 1932a, available on the FDA website. Failure to report undermines the FDA's ability to track safety signals across the compounding supply chain.

The 503A Versus 503B Distinction for Veterinary Practices

Section 503A pharmacies compound patient-specific prescriptions in response to a valid prescription from a licensed practitioner. Section 503B outsourcing facilities compound larger batches under current good manufacturing practice (cGMP) requirements and may distribute without patient-specific prescriptions.

For veterinary practices, the distinction matters when ordering office stock:

• 503A pharmacies may compound office stock only from bulk drug substances on the FDA's listed or under-review lists, and only in quantities consistent with the GFI #256 framework.
• 503B outsourcing facilities compound under cGMP and may provide office stock, but they still must comply with GFI #256's bulk drug substance requirements for animal drugs. The FDA has stated that 503B facilities are not exempt from the animal drug compounding framework.

What Changes in 2026 and Beyond

The FDA's implementation of GFI #256 continues to evolve:

• The agency is reviewing bulk drug substance nominations on a rolling basis and updating the office stock list periodically.
• GFI #256B, a draft guidance on compounding under cGMP in federally registered facilities, is under development and expected to be published by the end of 2026.
• The FDA hosted a compounding listening session in February 2026, with slides publicly available through the American College of Veterinary Pharmacists.
• New drug approvals continue to reduce the need for certain compounded products. For example, the FDA approved atinvicitinib (Numelvi) in 2026, adding a new approved option for conditions that previously may have required compounded alternatives.

Practices should check the FDA's bulk drug substance lists periodically and review their compounding pharmacy relationships annually.

Key Takeaways

• GFI #256 does not legalize bulk compounding — it describes when the FDA does not intend to enforce. Treat it as a compliance framework, not a permission slip.
• Office stock from bulk drug substances is limited to the FDA's listed and under-review substances. Anything else requires a patient-specific prescription with documented rationale.
• Your practice shares responsibility with the compounding pharmacy. Keep records, document medical rationale, and report adverse events.
• Evaluate compounding pharmacies for licensing, GFI #256 compliance, quality testing, and veterinary specialization before relying on them.
• When state and federal rules conflict, the more restrictive standard applies. Know your state's requirements.

Sources

• FDA CVM GFI #256 — Compounding Animal Drugs from Bulk Drug Substances. U.S. Food and Drug Administration. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/cvm-gfi-256-compounding-animal-drugs-bulk-drug-substances
• FDA Q&A: GFI #256 — Compounding Animal Drugs from Bulk Drug Substances. U.S. Food and Drug Administration (updated March 18, 2026). https://www.fda.gov/animal-veterinary/animal-drug-compounding/qa-gfi-256-compounding-animal-drugs-bulk-drug-substances
• FDA Animal Drug Compounding overview. U.S. Food and Drug Administration. https://www.fda.gov/animal-veterinary/unapproved-animal-drugs/animal-drug-compounding
• Federal Register notice: Compounding Animal Drugs From Bulk Drug Substances; Guidance for Industry; Availability (April 14, 2022). https://www.federalregister.gov/documents/2022/04/14/2022-08092/compounding-animal-drugs-from-bulk-drug-substances-guidance-for-industry-availability
• ACVP April 2026 Regulatory Roundup: FDA Compounding, New Approvals, and Stewardship. American College of Veterinary Pharmacists. https://vetmeds.org/fda-veterinary-updates-april-2026
• Wickliffe Veterinary Pharmacy — Understanding GFI-256. https://www.wicklifferx.com/understanding-gfi-256
• Wedgewood Pharmacy — GFI #256: Navigating What It Means Today. https://www.wedgewood.com/learning-center/gfi
• Dalcoma Specialty Pharmacy — GFI 256 FDA Guideline. https://www.dalcomarx.com/fda-guideline
• Quarles & Brady — FDA Publishes New Guidance on Compounding Animal Drugs from Bulk Drug Substances. https://www.quarles.com/newsroom/publications/fda-publishes-new-guidance-on-compounding-animal-drugs-from-bulk-drug-substances
• FDA Guidances Under Development — CVM 2026. https://www.fda.gov/animal-veterinary/guidance-industry/guidances-under-development