PARIS — For decades, American horses, many of them retired or damaged racehorses, have been shipped to Canada and Mexico, where it is legal to slaughter horses, and then processed and sold for consumption in Europe and beyond.

Lately, however, European food safety officials have notified Mexican and Canadian slaughterhouses of a growing concern: The meat of American racehorses may be too toxic to eat safely because the horses have been injected repeatedly with drugs.

Despite the fact that racehorses make up only a fraction of the trade in horse meat, the European officials have indicated that they may nonetheless require lifetime medication records for slaughter-bound horses from Canada and Mexico, and perhaps require them to be held on feedlots or some other holding area for six months before they are slaughtered.

In October, Stephan Giguere, the general manager of a major slaughterhouse in Quebec, said he turned away truckloads of horses coming from the United States because his clients were worried about potential drug issues. Mr. Giguere said he told his buyers to stay away from horses coming from American racetracks.