Major pet product retailer Chewy and eight other companies have been accused of selling unapproved animal medications.
The Food and Drug Administration leveled the accusations in a letter to the nine companies on Thursday.
The agency alleges that the company violated federal law by selling unapproved medicines containing the antibiotics amoxicillin and penicillin.
The products, targeted at fish and birds, included Aqua-Mox, Aqua-Zithro, Doxycycline and Fishbiotic, the agency said.
FDA Veterinary Medicine Director Tracey Forfa said those antibiotics could fuel the evolution of drug-resistant “super bugs” - posing a risk to humans as well as animals.
“Inappropriate use of medically important antimicrobials contributes to the development of antimicrobial resistance, which affects both human and animal health,” Ms Forfa said in a statement.
The nine companies were ordered to present plans to address violations within 15 days, or else legal action will be escalated. The companies are: American Aquarium Products, Aquanest Biotic, Aquarium Pharmacy, California Veterinary Supply, Chewy Inc., Kraft Drug, Midland Veterinary Services, Silver Lease and Valley Veterinary Clinic.