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He also decided to test the manufacturing leftovers of one of his company’s first antibiotics, which is now known as chlortetracycline.

In 1943, there was streptomycin, the first cure for tuberculosis, and on the heels of that came chloramphenicol, chlortetracycline, neomycin, erythromycin.

In interviews, another major producer, Foster Poultry Farms, acknowledged that it also has used the antibiotics chlortetracycline and penicillin selectively but not as part of standard feed.

The medicine—either penicillin, vancomycin, a combination of the two, or chlortetracycline—was given at doses comparable to those approved by the U.S.

In December 2011, it quietly rescinded its initial 35-year-old plan to withdraw approval for penicillin and the two tetracycline drugs — chlortetracycline and oxytetracycline — on the grounds that the proposal was outdated.