Jennifer Beckett of Franck's Pharmacy in Ocala, Fla., told The Associated Press in a statement that the business conducted an internal investigation that found "the strength of an ingredient in the medication was incorrect." The statement did not say what the ingredient was.
Beckett, who's the pharmacy's chief operating officer, said the pharmacy is cooperating with an investigation by state authorities and the Food and Drug Administration.
The horses from the Venezuelan-owned Lechuza polo team began crumpling to the ground shortly before Sunday's U.S. Open match was supposed to begin, shocking a crowd of well-heeled spectators at the International Polo Club Palm Beach in Wellington.
"On an order from a veterinarian, Franck's Pharmacy prepared medication that was used to treat the 21 horses on the Lechuza Polo team," Beckett said. "As soon as we learned of the tragic incident, we conducted an internal investigation."
She said the report has been given to state authorities.
Lechuza also issued a statement to AP acknowledging that a Florida veterinarian wrote the prescription for the pharmacy to create a compound similar to Biodyl, a French-made supplement that includes vitamins and minerals and is not approved for use in the United States.
"Only horses treated with the compound became sick and died within 3 hours of treatment," Lechuza said in the statement. "Other horses that were not treated remain healthy and normal."
Lechuza also said it was cooperating with authorities that include the State Department of Agriculture and Consumer Services and the Palm Beach County Sheriff's Office.
Biodyl contains a combination of vitamin B12, a form of selenium called sodium selenite and other minerals. It is made in France by Duluth, Ga.-based animal pharmaceutical firm Merial Ltd. and can be given to horses to help with exhaustion. It is widely used abroad, but not approved in the U.S.
Compound pharmacies can, among other things, add flavor, make substances into a powder or liquid or remove a certain compound that may have an adverse reaction in different animal species. Only in limited circumstances can they legally recreate a drug that is not approved in the U.S., according to the FDA.
Necropsies of the 21 horses found internal bleeding, some in the lungs, but offered no definitive clues to the cause of death.
FDA spokeswoman Siobhan DeLancey said compounding pharmacies cannot legally recreate existing drugs or supplements under patent. In most cases, they are also not allowed to recreate a medication that is not approved for use in the U.S.
On its Web site, the FDA says it generally defers to "state authorities regarding the day-to-day regulation of compounding by veterinarians and pharmacists."
However, the agency says it would "seriously consider enforcement action" if a pharmacy breaks federal law in compounding medications. It isn't yet clear Franck's broke the law.