The bill would have required a pharmacist to list each ingredient in the compound and certify the product with a signature.
But Carmelo Cinqueonce, executive vice president of the S.C. Pharmacy Association, said it was unclear what problem the bill, S-453, sought to solve.
The author of the polo-horse provision, Rep. Chip Limehouse, R-Charleston, said he was alarmed after 21 polo horses died in April in Wellington, Fla., after they were given supplements.
"I think it's a knee-jerk reaction to the unfortunate incident in Florida," Cinqueonce said, adding that it would do nothing to regulate pharmacists who fill orders in Georgia that are given to horses in polo matches right across the state line.
In Aiken County, nine polo clubs are recognized by the United States Polo Association and about 40 fields, according to the Aiken Polo Club's Web site. Members of the Aiken Polo Club could not be reached.
"Folks in compounding don't see why this is necessary or what it's really trying to achieve that already isn't standard practice," Cinqueonce said.
In South Carolina, a state board enforces standards set by the U.S. Pharmacopoeia, a public-health organization.
Sanford objected to a part of the bill that had nothing to do with polo horses.
The governor pointed to the provision that would grant only residents who reside within two miles of a livestock operation the right to appeal a state livestock permit. An appeal that was decided in court could then affect every state resident, undermining local control, Sanford warned.
State lawmakers will return June 16 to consider overriding the governor's vetoes.