Indigo carmine
A synthetic blue dye made from petroleum chemistry, used to colour sweets, ice cream and drinks a vivid blue.
Manufacturing impurities include aromatic amines that regulators have not yet fully identified or toxicologically assessed. EFSA flagged these as an open data gap in 2023 and treated them conservatively as potentially carcinogenic. Separately, the arsenic limits in current specifications were judged too high and have been recommended for reduction.
What is it?
Indigo carmine (also called indigotine) is a water-soluble synthetic blue dye belonging to the indigoid family. It is produced by sulfonating indigo, itself derived from petroleum-based organic chemistry. On a food label it appears as 'indigo carmine', 'indigotine' or 'E132'. Its CI name is Food Blue 1 and it is sold in the US as FD&C Blue No. 2.
What does it do?
It dissolves readily in water and imparts a clear blue to blue-green colour across a wide pH range. Because it is poorly absorbed in the gut most of it passes through unchanged, which is why it is also used in medicine as a diagnostic dye for kidney function tests.
Where you will see it
Confectionery such as jelly sweets, boiled sweets and sugar-coated chocolates; edible ices and ice cream; flavoured dairy desserts; water-based flavoured drinks and squashes; cake decorations and icing; some baked goods; pharmaceutical tablet coatings and capsule shells. On a UK label look for 'E132', 'indigo carmine' or 'indigotine'.
What the science says
Unidentified manufacturing impurities flagged as an open data gap
When EFSA examined samples of commercial E132 in its 2023 follow-up review, analysts detected several peaks of unsulfonated aromatic amines using gas chromatography but could not identify them. Only aniline was confirmed among these compounds. Because the identity of the remaining amines was unknown, EFSA's safety panel took the conservative position of treating them as if they have the same carcinogenic potential as aniline (IARC Group 2A, probable carcinogen in humans). EFSA called for manufacturers to identify the compounds and lower their concentrations. Until that data is supplied, a residual uncertainty remains about what those compounds are and what they do.
Multiple unsulfonated aromatic amine peaks were detected in commercial indigo carmine samples but could not be identified; EFSA treated them conservatively as potentially aniline-like for risk assessment purposes.
Aniline and aniline hydrochloride are classified as probably carcinogenic to humans (Group 2A) by IARC, based on strong mechanistic evidence and its structural similarity to established human carcinogens in the aromatic amine class.
Arsenic levels in specifications judged insufficient
EFSA's 2023 review found that actual arsenic concentrations measured in commercial batches of E132 were substantially lower than the maximum levels currently allowed under EU specifications. The panel concluded the permitted ceiling is too high and recommended it be lowered to reflect what manufacturers can realistically achieve. When EFSA modelled current intake against the measured arsenic levels, the margin of exposure was below the threshold EFSA considers adequate for a substance that cannot be classified as a threshold carcinogen.
For arsenic, calculated margin of exposure values in both measured and maximum-permitted-level scenarios fell below the EFSA target of 1,000, indicating the current specification limit for arsenic should be revised downward.
Contested 1985 rat brain tumour finding
A two-year carcinogenicity study in rats (Borzelleca et al., published 1985) found a statistically significant increase in brain gliomas in high-dose male animals. Regulators, including the US FDA and later EFSA, concluded the finding was not biologically significant because glioma incidence fell within the typical historical range for the rat strain used and none of the other accepted criteria for a neurotoxic carcinogen were met. However, independent expert reviewers at the time disagreed, arguing the glioma data could not simply be dismissed. EFSA's 2014 opinion noted the ADI was set using data from the same research group and relied on material of comparable purity.
A chronic toxicity and carcinogenicity rat study found a statistically significant increase in brain gliomas in high-dose male animals; the FDA concluded this was not biologically significant on the basis that incidence remained within historical control ranges.
Independent expert reviewers Knezevich and Hogan concluded that the glioma findings in the 1985 Blue No. 2 rat study 'cannot be dismissed as accidental', contrasting with the FDA's conclusion.
Testicular effects in one animal study, linked to impurities not the dye itself
A 2013 mouse study (Dixit and Goyal) found adverse effects on testicular tissue at a dose of 17mg/kg bw/day, which EFSA's 2014 review flagged as a data gap requiring follow-up. A new 56-day dietary study using higher-purity material (88% purity) was submitted to EFSA and the results did not reproduce the testicular damage seen earlier. The panel concluded the original finding was most likely caused by impurities or contaminants in the lower-purity material, not by indigo carmine itself.
A 2013 mouse subacute toxicity study found a lowest observed adverse effect level for testicular damage at 17mg/kg bw/day; EFSA judged this required confirmation before the ADI could be considered secure.
A new 56-day dietary study using 88% pure material did not confirm the testicular effects; the panel concluded the adverse effects observed previously were attributable to impurities in the test material rather than to indigo carmine itself.
Genotoxicity: no concern from regulators
Multiple genotoxicity studies have been conducted on indigo carmine, including in vivo micronucleus tests. EFSA's 2014 and 2023 reviews both concluded the additive does not raise concern for genotoxic potential. Some older in vitro Salmonella assays produced positive results in the presence of a metabolic activation system, but these were not considered sufficient to indicate a genotoxic hazard given the overall data set.
Based on available data including a newly performed in vivo micronucleus study, indigo carmine does not raise concern for genotoxicity.
Rare allergic and hypersensitivity reactions
A small number of case reports in medical literature describe allergic reactions to indigo carmine, ranging from urticaria (hives) to anaphylactic-type reactions. The reactions are documented mainly in clinical settings where indigo carmine has been administered intravenously in large doses for diagnostic purposes, not at food-level oral doses. Some evidence links the reactions to sulfonamide hypersensitivity. At the concentrations typically found in food, allergic reactions are considered rare.
Case reports describe rare but severe anaphylactoid reactions including hypotension, severe hypoxia and urticaria following intravenous administration of indigo carmine in surgical settings, with cross-reactivity to sulfonamides suggested.
Where it stands with the regulators
Who should be careful
People with a known sulfonamide allergy should be aware of possible cross-reactivity, though food-dose reactions are rare. Indigo carmine is not permitted in foods made specifically for infants and young children. Look for 'E132', 'indigo carmine' or 'indigotine' on the label.
The honest read
The dye itself has been in the food supply for decades and EFSA's latest full review (2023) confirmed the ADI and found no concern at typical dietary exposure. But two things remain genuinely unsettled. First, the batch of commercial E132 tested in 2023 contained several aromatic amine compounds that analysts could not identify, and EFSA treated them, conservatively, as potentially as harmful as aniline, which IARC classes as a probable human carcinogen. Those compounds have not been identified or quantified to regulators' satisfaction. Second, the arsenic ceiling in current manufacturing specifications was judged too permissive. Neither issue has been formally resolved: EFSA has asked for the data; manufacturers have not yet supplied it. Whether the amounts reaching consumers are meaningful is unknown until that data arrives. The contested 1985 rat glioma study adds a further unresolved thread, though its significance has been debated by experts for four decades without consensus.
Related additives
Common questions
Is E132 banned in the UK?
No. Indigo carmine (E132) is approved for use in the UK under the assimilated version of EU Regulation (EC) No 1333/2008 and appears on the FSA's approved-additives list. It is not permitted in infant formula or foods made specifically for young children, where exposure levels would exceed the acceptable daily intake.
Did regulators find any unresolved concerns with E132?
Yes, two. EFSA's 2023 review found that commercial samples of E132 contained several aromatic amines that could not be identified, and EFSA conservatively treated these as potentially carcinogenic (similar to aniline, IARC Group 2A). EFSA also found the permitted arsenic limits in the manufacturing specification too high and recommended they be lowered. Neither issue has been resolved yet because the requested manufacturer data had not been supplied at the time of the opinion.
What foods contain E132?
Blue-coloured sweets, jelly sweets, sugar-coated confectionery, ice cream, edible ices, flavoured soft drinks, cake decorations and icings. It also appears in pharmaceutical tablet coatings and capsule shells. On a label look for 'E132', 'indigo carmine' or 'indigotine'.
Is E132 vegan?
Yes. Indigo carmine is produced synthetically from petroleum-derived chemistry. No animal or insect material is used in its manufacture, so it is suitable for vegans and vegetarians.
Sources
- EFSA FAF Panel: Follow-up of the re-evaluation of indigo carmine (E 132) as a food additive (2023)
- EFSA ANS Panel: Scientific Opinion on the re-evaluation of Indigo Carmine (E 132) as a food additive (2014)
- PMC full text of EFSA 2023 follow-up opinion (PubMed Central)
- IARC Monographs Volume 127: Some Aromatic Amines and Related Compounds (aniline Group 2A classification)
- UK FSA Approved Additives and E Numbers (E132 listed)
- UK FSA Regulated Products Register: E 132
- Borzelleca JF et al. Chronic toxicity/carcinogenicity study of FD&C Blue No. 2 in rats (PubMed abstract)
- IARC Aniline and Aniline Hydrochloride - NCBI Bookshelf extract
- Regulation (EC) No 1333/2008 on food additives (including Annex V children's warning label) as assimilated UK law
Aaron Keen is the founder of NutraSafe. He researches and writes every additive entry himself, from the primary sources. About the research →
This is a guide, not medical advice. If an additive affects you, speak to your GP or a dietitian.
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