More than two years since the U.S. Food and Drug Administration proposed a way to handle a perennially sticky aspect of veterinary drug compounding — when to allow the use of bulk ingredients — the regulatory agency has backed off the plan.

The FDA announced Tuesday that it is withdrawing its draft guidance for industry, "Compounding Animal Drugs from Bulk Drug Substances," also referred to as GFI #230. The guidance was something the agency promised in 2004 would be forthcoming.

But the agency did not finalize the guidance, and it won't. Instead, the FDA said it will issue a new draft for public comment early next year.

Public comments on the draft guidance issued in May 2015 apparently led the agency to change course, but which comments were pivotal is unclear. The announcement says only, "After reviewing the comments submitted to the docket, the FDA decided not to finalize the current draft guidance, and will instead develop and issue a new draft guidance. In developing the new draft, the FDA will carefully consider the issues that are specific to compounding of animal drugs, including the significance of using compounded drugs as a treatment option in various veterinary settings and animal species."

Distinct from pharmaceutical manufacturing, compounding is a practice by which drugs are changed in dosage, form and/or flavor to fit the needs of a particular patient. Federal law does not permit the compounding of animal drugs from raw, or bulk, ingredients, only from FDA-approved finished drugs.

In certain instances, however, the FDA will look the other way when bulk substances are used. The agency states: "The FDA recognizes that there are circumstances where there is no approved drug that can be used or modified through compounding to treat a particular animal with a particular condition. In those limited situations, an animal drug compounded from bulk drug substances may be an appropriate treatment option."

The guidance being withdrawn proposed conditions under which the agency probably would not take enforcement action. The document was intended to clarify the exemptions for veterinary compounding.

A reading of the more than 100 comments submitted by veterinarians, veterinary associations, pharmaceutical companies, compounding pharmacists, pet owners and others suggests that the draft rendered the situation anything but clear.

A number of veterinarians and pet owners interpreted the proposed guidelines to mean that compounded preparations would become unavailable or be severely restricted for animal patients.

Other comments, primarily those of pharmaceutical manufacturers and their professional organizations, expressed that the FDA has done too little in the realm of veterinary compounding.

At the other end of the spectrum, commenters including several compounding pharmacies questioned the agency's authority to oversee drug compounding at all.

These are well-established views in a long-standing debate and discussion about veterinary compounding and its legality.

But some commenters did address particular aspects of the proposal rather than broad themes. A number of veterinary organizations, for instance, objected to the FDA’s use of a list of specific drugs that could be exempted from the prohibition against the use of bulk ingredients. The commenters said using such a list is an inflexible approach, not easy to update. The American Association of Equine Practitioners requested that the agency instead specify the circumstances under which medications could be compounded from bulk ingredients.

Until the next draft guidance is issued, the FDA said veterinarians and other stakeholders with questions about animal-drug compounding may contact the agency’s Center for Veterinary Medicine at AskCVM@fda.hhs.gov.