E-numbers / Preservatives / E233
also: Tiabendazole · TBZ · 2-(thiazol-4-yl)benzimidazole
A synthetic antifungal used as a post-harvest coating on citrus fruit and bananas to prevent mould. Not permitted as a food additive in the UK or EU.
Scan a barcode to check a food for thiabendazole. NutraSafe Pro · £6.99/month · iOS
EFSA confirmed in 2022 that thiabendazole meets the criteria for endocrine disruption in the thyroid.
Why it’s worth knowing. EFSA confirmed in 2022 that thiabendazole meets the criteria for endocrine disruption in the thyroid. Animal studies show thyroid follicular changes and liver effects at elevated doses. The US EPA classifies it as likely carcinogenic at doses high enough to disturb thyroid hormone balance.
Thiabendazole is a synthetic benzimidazole compound originally developed as an antiparasitic drug and later adopted as an antifungal preservative. It inhibits cell division in fungi by binding to tubulin, the protein that forms the cell's internal skeleton. It is also used as a veterinary medicine to treat parasitic worms.
Applied to the surface of fruit after harvest, thiabendazole disrupts fungal cell division by interfering with tubulin polymerisation, preventing moulds from forming and extending shelf life during long-distance shipping and cold storage. It does not penetrate deeply into the fruit flesh when applied to peel.
Historically applied as a post-harvest wax coating or dip on citrus fruits (oranges, lemons, grapefruit, limes) and bananas. It is not used as an ingredient inside processed or packaged food. Under UK and EU food additive rules, E233 is no longer permitted, but thiabendazole residues may still appear on imported citrus from countries where it is approved. When the outer peel of treated fruit has been used as a food ingredient (for example in marmalade or cocktails), the label was required to declare the treatment.
EFSA completed a peer review in 2022 and concluded that thiabendazole meets the EU regulatory criteria for endocrine disruption in the thyroid. Animal studies consistently showed elevated thyroid-stimulating hormone, reduced triiodothyronine (T3), and follicular cell changes in the thyroid across multiple species and study designs. This was designated a critical area of concern under EU pesticide regulation.
Thiabendazole meets the criteria for endocrine disruption for humans in the thyroid modality, constituting a critical area of concern under Annex II of Regulation 1107/2009.
Rat and dog studies showed elevated TSH, reduced T3 and increased thyroid weight at doses of 90 mg/kg per day and above, consistent with indirect stimulation of the thyroid gland leading to follicular cell proliferation.
The US EPA classifies thiabendazole as likely to be carcinogenic to humans at doses high enough to disturb thyroid hormone balance, but not at lower doses. The carcinogenicity signal is mechanistically linked to the thyroid disruption rather than direct DNA damage. Thyroid follicular cell adenomas were observed in rat carcinogenicity studies and were considered part of the endocrine disruption evidence by EFSA.
The US EPA classifies thiabendazole as likely to be carcinogenic to humans at doses sufficient to cause a disturbance of thyroid hormone balance, but not likely to be carcinogenic at lower doses.
Thyroid follicular cell adenomas observed in rat carcinogenicity studies were cited by EFSA as part of the thyroid-mediated endocrine disruption evidence base.
Studies in rats and dogs showed liver weight increases and signs of haemolytic anaemia at elevated doses. JECFA identified haematotoxicity (bone marrow and spleen changes consistent with anaemia) as the most sensitive endpoint and used it to set the ADI.
In rat and dog studies, histopathological changes in the spleen and bone marrow indicative of anaemia were the most sensitive endpoint identified, with a no-observable-effect level of 9 to 10 mg/kg per day.
Liver hypertrophy and thyroid follicular cell hyperplasia were observed in rat subchronic studies at doses of 37 mg/kg per day and above.
High-dose animal studies found skeletal malformations in mice and embryotoxicity in rabbits. These effects occurred at doses well above food residue exposure levels. JECFA set an overall developmental no-effect level at 24 mg/kg per day based on findings in rabbits and mice.
Developmental toxicity including skeletal malformations in mice and increased fetal resorption in rabbits was observed at high doses. The overall no-effect level for developmental effects was 24 mg/kg per day.
Regulatory assessments by JECFA and the EU Pesticide Properties Database have recorded negative results in standard genotoxicity tests including bacterial mutation assays and mouse bone marrow tests. However, more recent in-vitro studies have found DNA strand breaks in human lymphocytes and mutagenic activity under UV light exposure, and one 2025 study reported mutagenic effects in bacterial testing. The evidence is conflicting and the regulatory consensus has not yet incorporated the newer findings.
Standard genotoxicity tests including chromosome aberration, DNA damage, and gene mutation assays returned negative results in regulatory evaluations.
A 2025 study found thiabendazole induced frameshift and base-pair mutations in bacterial Ames testing and caused dose-dependent DNA strand breaks in human lymphocytes in a comet assay.
Thiabendazole concentrates in the peel. UK pesticide monitoring found thiabendazole in grapefruit at 2.4 mg/kg in 2024. The UK Health and Safety Executive calculated that peeling the fruit before eating leaves only 5% of the residue in the flesh, bringing intake within safety margins. Eating peel on its own, or for infants specifically, brings exposure closer to the acute reference dose.
In Q1 2024, the UK pesticide residues programme detected thiabendazole in grapefruit at 2.4 mg/kg. HSE risk assessment found no exceedance of the ARfD when the peel is removed, but infant exposure including peel reached 131% of the ARfD in a worst-case analysis.
People who use the zest or peel of non-organic citrus fruit regularly (for example in cooking, baking or drinks) may have higher exposure because thiabendazole concentrates in the peel. Those preparing food for infants using citrus peel should be aware that worst-case exposure estimates for this group exceed the acute reference dose. Look for 'unwaxed' on citrus labels; choose organic citrus if using peel. The substance may still appear on the label of treated citrus as 'thiabendazole' or 'E233' if declared.
The picture on thiabendazole is not straightforward. As a food additive it was quietly dropped from the EU and UK approved lists. Yet it still reaches UK shoppers as a pesticide residue on imported citrus, within legal limits. The endocrine disruption finding in 2022 was significant: EFSA designated it a critical area of concern, a conclusion that underpins ongoing pressure to tighten or remove its pesticide approvals in Europe. The carcinogenicity classification from the US EPA is threshold-dependent and tied to the thyroid disruption, not to direct DNA damage, though newer in-vitro research raises some uncertainty about genotoxicity that has not yet been resolved by regulators. For most people peeling their citrus before eating, exposure to the flesh is low. For anyone regularly cooking with citrus zest or peel from non-organic fruit, the picture is less clear.
As a food additive, yes. Thiabendazole (E233) does not appear on the UK FSA approved additives list or in EU Regulation 1333/2008 Annex II (the regulation retained in UK law). It is not a lawful food additive in UK food. However, it is still permitted as a post-harvest pesticide fungicide on citrus, subject to maximum residue levels, under separate UK and EU pesticide law.
Yes, formally. EFSA concluded in 2022 that thiabendazole meets EU regulatory criteria for endocrine disruption in the thyroid. The evidence includes consistent findings of elevated thyroid-stimulating hormone, reduced T3, and thyroid follicular cell changes across multiple animal studies. EFSA designated this a critical area of concern for its ongoing pesticide approval.
As an authorised food additive, none in the UK or EU. As a pesticide residue, thiabendazole may be present on the peel of imported citrus fruit (oranges, lemons, grapefruit, limes) and bananas where it is applied post-harvest to prevent mould. UK monitoring in 2024 found it in grapefruit samples. It is not present in processed foods as an ingredient.
Thiabendazole itself is a synthetically produced chemical compound and does not derive from animal products. On that basis it is considered vegan. However, some fruit wax formulations used alongside thiabendazole may contain shellac or beeswax; these are separate additives and would need to be checked independently.
Last reviewed: 20 June 2026
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