The paper, entitled "Association of phenylbutazone usage with horses bought for slaughter: A public health risk", appeared in the journal Food and Chemical Toxicology.
The paper, by Drs Nicholas Dodman, Nicolas Blondeau and Ann Marini, followed 18 thoroughbreds whose drugs records showed they had been given phenylbutazone, also known as bute. The horses were subsequently sent for slaughter.
The study also traced records of 16 thoroughbreds that were given the drug on race day and would have also entered the food chain had they not been rescued.
The study was limited to racehorses because of the availability of drug records.
"The permissive allowance of such horsemeat used for human consumption poses a serious public health risk," the authors said, noting that 67 million pounds of horsemeat derived from American horses were sent abroad for human consumption last year.
"Horses are not raised as food animals in the United States and, mechanisms to ensure the removal of horses treated with banned substances from the food chain are inadequate at best," they said.
Phenylbutazone is one of the most common drugs used in the treatment of musculoskeletal injuries in horses.
Because of the bone marrow toxicity caused by phenylbutzone in humans, the US Food and Drug Administration (FDA) has set no safe levels of the drug and bans its use food-producing animals, including horses.
The Equine Welfare Alliance (EWA), which has long pointed out the likelihood that meat contaminated with the drug is entering the human food chain, said the study called into question the reliability of US Department of Agriculture (USDA) and Canadian Food Inspection Agency (CFIA) testing programmes, which have consistently failed to detect the substance.
"Defenders of horse slaughter have long pointed to USDA testing records which consistently showed no positive results for PBZ," said EWA representatives John Holland and Vicki Tobin.
"The new study shows that the USDA testing could not have been accurate.
"Opponents of horse slaughter have long warned that US horses are not raised as food animals and mechanisms to ensure the removal of horses treated with banned substances from the food chain are inadequate at best," they said.
The European Union is bringing in traceability requirements for horses entering the food chain in a bid to ensure drugs such as bute do not enter the food chain. The move is likely to impact heavily on the American slaughter industry, centred on Canada and Mexico.