Nitrofurans are a class of antibacterial agents comprised of nitrofural/nitrofurazone, furazolidone and furaltadone, which were used in the past for treating infectious diseases in animals.
 
In recent years, there is a widespread concern about nitrofurans in society since some evidences have been revealed that nitrofurans may cause cancer in animals. Many studies on nitrofural/nitrofurazone show that SEM (semicarbazide), a metabolite of nitrofural/nitrofurazone, may occur naturally in some food (e.g. crustaceans like crayfish) or may form in food processing. The presence of SEM in food may be also caused by the residues of veterinary drugs in food animals. Laws have been enacted in mainland China, the European Union, the United States, Macao SAR and other regions to prohibit the use of nitrofurans in food animals.
 
Reference Source:
EFSA Journal (2015) of the European Food Safety Authority: Scientific Opinion on nitrofurans and their metabolites in food.