Research conducted by the American Cyanamid laboratories in the late 1940s and early 1950s demonstrated that adding penicillin to chicks' feed increased their weight gain by 10 per cent. The reasons for this were still subject to debate in the twenty-first century. Subsequent research indicated that adding penicillin to animal feed also improved feed conversion efficiency, promoted more uniform growth and facilitated disease control. After the Food and Drug Administration approved the use of penicillin as feed additives for poultry and livestock in 1951, the pharmaceutical companies ramped up production to meet the demand.
By 1954, the United States was producing  of antibiotics each year, of which  was going into animal feed; in the 1990s, the United States was producing  of antibiotics per year, of which half was going to livestock. The largest user remained the poultry industry, which consumed  of antibiotics each year, compared to  for hogs and  for cattle. A 1981 study by the Council for Agricultural Science and Technology estimated that banning their use in animal feed could cost American consumers up to $3.5 billion a year (equivalent to $ billion in ) in increased food prices. The story was similar in the UK, where 44 per cent of antibiotic production was consumed by animals by 1963.
By the mid-1950s, there were reports in the United States that milk was not curdling to make cheese. The FDA found that the milk was contaminated with penicillin, which was killing the bacteria required for cheesemaking. In 1963 the World Health Organization reported high levels of penicillin in milk worldwide. People who were allergic to penicillin could now get a reaction from drinking milk. A committee chaired by Lord Netherthorpe was established in the UK in 1960 to inquire into the use of antibiotics in animal feed. In 1962, the committee recommended that restrictions on the use of antibiotics in animals be relaxed. It contended that the benefits were substantial and that even if bacteria became resistant, new antibiotics would soon be developed, and there was no evidence that bacterial resistance in animals impacted human health.
The Netherthorpe committees conclusions were undermined by new research even before they were published, and the committee was recalled to reconsider the issue in 1965. New studies had shown that bacteria were not only were able to inherit the genes for antibiotic resistance, but they could also communicate them to each other. In 1967, a multiresistant strain of E. coli' killed fifteen children in the UK. The use of antibiotics in animals for nontherapeutic use was banned there in 1971. Many other European countries soon followed.
When Sweden acceded to the European Union (EU) in 1995, a total ban on antibiotic growth promoters (AGPs) had been in place there for ten years. This would be superseded by more relaxed EU rules unless Sweden could demonstrate scientific evidence in favour of a ban. Two Swedish scientists, Anders Franklin and Christina Greko, and two Danish scientists,  and , took up the fight. The odds seemed against them but this coincided with the United Kingdom BSE outbreak, which resulted in intense political pressure. In December 1996, the European Parliament's Standing Committee on Health and Welfare voted to ban the use of AGPs. The EU went further and recommended broad restrictions on the use of antibiotics.