Franck's Pharmacy's latest disclosure follows the announcement by Florida state veterinarian Thomas Holt that the 21 horses that died before a US Open polo match 12 days ago had "significantly increased selenium levels".
An overdose of selenium was the probable cause of the deaths, he said.
Franck's Pharmacy chief operations officer Jennifer Beckett said: "As we said last week, the strength of an ingredient in a medication Franck's Pharmacy prepared for the 21 horses on the Lechuza Polo team was incorrect.
"In light of today's statement from Florida State Veterinarian Thomas J. Holt, we can confirm that the ingredient was selenium," she said.
"We continue to co-operate fully with the authorities as their investigations proceed. Because of the ongoing investigations, we cannot discuss further details about this matter at this time."
Blackett's earlier statement, on April 23, said as soon as the pharmacy learned of the deaths, it conducted an internal investigation led by an outside lawyer and, upon its conclusion, 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.
Around that time, Lechuza Polo confirmed 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.
The compounded medication was to contain vitamin B, potassium, magnesium and selenium, it said.
"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."
Meanwhile, animal health company Merial has released a statement over earlier speculation in some media over its product Biodyl.
"Some early media reports speculated as to whether Merial's product Biodyl may have been among the pharmaceutical preparations used with the horses," said spokesman Steve Dickinson.
"This was not the case. Merial's Biodyl is not approved by the FDA for use in the United States, and was not given to the Palm Beach horses. Rather, the veterinary pharmacy reportedly attempted to compound a substance meant to recreate genuine Biodyl.
"Merial has not been contacted by any officials involved in the investigation," he said.
"Biodyl is approved for sale in many other countries of the world, where it has been used safely for decades. We are confident that Merial's Biodyl is safe when used as directed."
Dickinson said the company is dedicated to the health and wellbeing of animals. "We are deeply saddened by the loss of these horses."