Franck's Pharmacy chief operations officer Jennifer Beckett said the strength of an ingredient in the preparation was incorrect. The ingredient has not been named.
"As soon as we learned of the tragic incident, we conducted an internal investigation that was led by an outside lawyer and, upon its conclusion, we immediately alerted the state Department of Health and Board of Pharmacy.
"The report, which we are furnishing to these agencies, concluded that the strength of an ingredient in the medication was incorrect," she said.
"We will co-operate fully with the authorities as they continue their investigations."
Beckett said Franck's Pharmacy has been providing patients, doctors and veterinarians with custom medications for more than 25 years.
"We extend our most sincere condolences to the horses' owners, the Lechuza Polo team and the members of the United States Polo Association. We share their grief and sadness," she said.
"Because of the ongoing investigations, we cannot discuss further details about this matter at this time."
Lechuza Polo released a statement confirming that a Florida-licensed veterinarian wrote a prescription for a compounded vitamin supplement intended as a substitute for a branded medication which is made in France.
"This compound was prepared in the State of Florida by a compounding pharmacy. Only the horses treated with the compound became sick and died within three hours of treatment. The horses that were not treated remain healthy and normal.
"Lechuza will continue co-operating with the authorities. We are committed to determining the actual cause and it is our hope that medical experts can do so in a timely and conclusive manner."
The team said there had been much conjecture and speculation in the media over the deaths.
"We remain deeply saddened by this tragic event. The investigation into the tragedy, however, has just begun and it will take medical experts days if not weeks to decide decisively the cause of the deaths.
"A definitive diagnosis can only be made when all of the facts are known."