Total sales of ionophore antibiotics, the most widely used antibiotics in UK farming, increased by 21% in 2023 compared with 2022, according to government data obtained by the Alliance to Save Our Antibiotics via a Freedom of Information request and published in a new report, “Ionophores – the most widely used and least regulated farm antibiotics”.
In 2023, 270 tonnes of ionophores were sold for use in poultry. This is 17 times higher than the use of all medically important antibiotics in poultry (15 tonnes) and more than the total sales of all other antibiotics in all animal species (189 tonnes) [1].
Despite their widespread use, ionophores are excluded from the veterinary antibiotic-sales data, published by the government’s Veterinary Medicines Directorate. This is because these drugs are animal-only antibiotics, as they are too toxic to be used in human medicine. They are therefore less regulated than other antibiotics used in farming.
Ionophores are routinely added to chicken feed, without the need for a veterinary prescription, to prevent a disease called coccidiosis. Coccidiosis is the most important disease problem in intensive chicken farming and occurs when chickens ingest chicken droppings. This happens frequently in intensive-farming systems, where sheds can contain tens of thousands of birds, kept at a stocking rate of about 19 birds per square metre [2].
It is often claimed that high ionophore use has no consequences for antibiotic resistance in human medicine, because ionophores are not related to antibiotics used in humans.
However, recent studies provide strong evidence that the use of ionophores in poultry does in fact increase resistance to antibiotics used in human medicine in enterococci bacteria in poultry, which can transmit to humans. Enterococci are bacteria which can cause serious infections in humans.
The rise of antibiotic resistance is one of the top global public health threats [3], and is directly responsible for 1.14 million deaths and associated with 4.71 million deaths a year [4]. World Antibiotic Awareness week is 18 to 24 November.
Cóilín Nunan of the Alliance to Save Our Antibiotics said: “Several studies have now found consistent evidence that widespread ionophore use in poultry is selecting for resistance to medically important antibiotics. UK regulators need to take action to end the routine use of ionophores in UK poultry. This will help prevent resistant bacteria spreading to humans through food.
In Norway, most poultry farmers stopped using ionophores in 2016, when this evidence had begun to emerge. Subsequently resistance to an antibiotic that is critically important antibiotic in human medicine fell sharply in Norwegian poultry [5].”
In research published earlier this year, US scientists investigated the global distribution of genes which make enterococci resistant to ionophores. They found they were present in bacteria from both farm animals and humans, and were linked with many genes conferring resistance to medically important antibiotics, including some classified as critically important. The scientists said: “There is thus clear potential for ionophore use to impact the presence of antibiotic resistance genes in the food supply.” [6]
Similarly, in 2022, Dutch scientists found that ionophore resistance genes were genetically linked to other genes conferring resistance to several medically important antibiotics in enterococci from poultry. They said: “This is an alarming observation, since it implies that the use of ionophores may drive the transfer and dissemination of other clinically relevant types of antimicrobial resistance by co-selection.” [7]
The Dutch scientists warned that abandoning the preventative use of ionophores would be “inevitable” and said that coccidiosis would need to be managed by other means, such as vaccination, which is now widely used in Norway.
In organic farming, preventative ionophore use has never been permitted and coccidiosis is controlled by giving chickens much more space and by allowing them access to the outdoors. Better health is also achieved by using slower-growing breeds of chickens.
Cóilín Nunan said: “Ending routine preventative ionophore use would mean that hygiene standards on chicken farms would need to be improved, so that chickens were no longer ingesting chicken droppings. This would mean keeping fewer birds per square metre. The cost implications would not be huge. One industry analysis estimates it would cost just 6p-9p extra per bird, whereas another estimated it would be about 11p.” [8][9]
Because of their high toxicity, there are also concerns about residues of ionophores in food, which are sometimes found in hen eggs, in pheasant and partridge meat, and in quail eggs.
Residues of ionophores and their metabolites in excreta produced by chickens can also contaminate soil and surface waters when chicken manure is spread on land.
In 2024, scientific opinions were published by a scientific panel of the European Food Safety Authority on the safety and efficacy of two ionophores, monensin and narasin, which are both used in EU and UK poultry production. In both cases, the scientific panel was unable to conclude that the drugs were not environmentally harmful. It said it could not exclude that monensin posed an environmental risk to the “aquatic compartment” or exclude that narasin posed a risk to the “terrestrial compartment” [10][11].
The UK poultry industry has made welcome cuts to its use of medically important antibiotics, which has fallen by 83% since 2012 [12]. However, its far greater use of ionophores has increased by 27% over the same period [13], as intensive-farming practices have not significantly improved.
Routine use of medically important antibiotics in all farm animals was banned in May 2024 [14]. The Alliance to Save Our Antibiotics believes that this ban should now be extended to ionophores, which should become prescription-only medicines.
To enable the phase out of routine ionophore use, improved husbandry is needed. It should no longer be permitted to keep chickens and other poultry in conditions that are so cramped and unhygienic that disease becomes unavoidable and routine use of medication is required. The maximum stocking rate permitted should therefore be reduced.
Even before legislation is implemented, UK poultry farmers should follow the example set by Norwegian poultry farmers, and work towards voluntarily phasing out ionophore use.
The Alliance to Save Our Antibiotics is an alliance of health, medical, civil-society and animal-welfare groups campaigning to stop the overuse of antibiotics in animal farming. It was founded by Compassion in World Farming, the Soil Association and Sustain in 2009.
Notes to Editors
[1] Data on the sales of medically important antibiotics are published annually by the Veterinary Medicines Directorate, https://www.gov.uk/government/publications/veterinary-antimicrobial-resistance-and-sales-surveillance-2023 However, data on the sales of ionophore antibiotics are not included in this report as they are not considered medically important.
[2] RSPCA, https://www.rspca.org.uk/adviceandwelfare/farm/meatchickens
[3] WHO, 2023. https://www.who.int/news-room/fact-sheets/detail/antimicrobial-resistance
[4] Antimicrobial Resistance Collaborators, 2023. https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(21)02724-0/fulltext
[5] Simm et al., 2019. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0226101
[6] Ibrahim et al., 2025. https://journals.asm.org/doi/epub/10.1128/msphere.00243-25
[7] Pikkemaat et al., 2022. https://edepot.wur.nl/565488
[8] Gittins et al., 2021. https://www.tandfonline.com/doi/pdf/10.1080/00439339.2022.1988807
[9] Parker et al., 2020. https://bvajournals.onlinelibrary.wiley.com/doi/full/10.1002/vetr.513
[10] EFSA, 2024. https://www.efsa.europa.eu/en/efsajournal/pub/8628
[11] EFSA, 2024. https://www.efsa.europa.eu/en/efsajournal/pub/8613
[12] British Poultry Council, 2025. https://britishpoultry.org.uk/bpc-antibiotic-stewardship-report-2025-designing-for-impact/
[13] Data obtained by FOI requests submitted by the Alliance to Save Our Antibiotics to the Veterinary Medicines Directorate
[14 The Veterinary Medicines (Amendment etc.) Regulations 2024, https://www.legislation.gov.uk/uksi/2024/567/contents/made
