I've heard that but I eat them anyway. It's not an antibiotic in there in spite of being called "medicated." It's normally Amprolium (sp?)
From University of Penn website:
Amprolium
Chemical group: thiamine analog
Trade names: Corid, Amprol
Mode of action: thiamine analog, competitively inhibits the active transport of thiamine. The coccidia are 50 times as sensitive to this inhibition as is the host.
Pharmacokinetics: Freely soluble in water.
Dose: In dogs and cats: "extra label use": 300 to 400 mg/kg, SID for 5 days or 110 to 220 mg/kg SID for 7 to 12 days.
In chickens: For prevention of coccidiosis: 36.3 - 113.5 g/ton in the feed or in the drinking water at 0.012%. No withdrawal is needed.
Toxicity: May cause thiamine deficiency if given at high rates.
Parasite Targets: Isospora in dogs and cats, Eimeria in cattle and poultry.
Selected references: Lindsy and Blagburn, (1995), Chap. 47, Antiprotozoan drugs. IN: Veterinary Pharmacology and Therapeutics, ( pg. 975), H. Richard Adams (ed). Iowa State Press, Ames, Iowa.