The association's decision followed the deaths earlier this year of 21 polo ponies in Florida, given an incorrectly mixed formulation which contained too much selenium.
The Humane Society of the United States said it commended the polo association for its drug-testing move.
The society's chief innovations officer, Holly Hazard, said the society had called for a mandatory drug testing protocol to be implemented in the wake of the deaths.
"We urge that adequate penalties also be implemented to serve as a strong deterrent to any drugging that could endanger equine welfare," she said.
"In all equestrian disciplines, the welfare of the equine participants must be of utmost concern. This is a positive step toward providing better protections for the equine competitors who participate in this rigorous sport."