Sulfonamides are structural analogs of para-aminobenzoic acid (PABA) and thus act as competitive antagonists in microbial cells. Sulfonamides are widely used as feed additives, mainly for fattening of calves and pigs. The presence of sulfonamide (SA) residues in foods is largely due to the raising of animals with sulfonamide antibiotics added or polluted feedstuff. Combined with inhibitors of dihydrofolate reductase such as trimethoprim, tetroxoprim, or pyrimethamine sulfonamides are also used in veterinary medicine for the treatment of intestinal infections, mastitis, pulmonitis and other (systemic) diseases.