Products commonly given to horses including dewormers are often banned for food animals.
More than 100,000 American horses are transported each year to Canada and Mexico to be slaughtered for their meat, most of it bound for European consumption.
The site points to concerns that racehorses, show horses, trail horses, companions and pets that enter the slaughter chain may well be unsuitable for human consumption due to previous use of banned medications, including phenylbutazone, or bute.
"There is now a document for Canada and Mexico that requires owners to disclose medications that are banned for horses intended for food," the website says, "but this does not mean that all of the information will be accurate.
"Even though Canada and Mexico implemented policy to comply with the European Union requirements, there is contention that the policy is doing little or nothing to ensure the horses that are being slaughtered today are not contaminated with prohibited drugs."
The website includes a discussion forum.