Last reviewed: 8 May 2026
The blue food dye that's also injected into patients during lymph-node biopsy
Patent Blue V (E131) is unusual among UK food additives. The same molecule that turns up in some confectionery and cake decorations is licensed as a medical imaging agent — surgeons inject it during sentinel-lymph-node biopsy to map the lymphatic drainage of breast tumours and melanomas. That medical use has produced the most rigorous published data we have on E131-related hypersensitivity, and it shapes how a UK shopper should think about the food-additive form. We walk through what E131 actually is, where it appears on UK labels, what the surgical literature tells us, why E131 is structurally different from the Southampton Six dyes, and where the regulatory picture sits in the UK versus other jurisdictions.
E131 is a synthetic triarylmethane dye — a class of colours built around a central carbon atom bonded to three aromatic ring systems. The full chemical name is the calcium or sodium salt of [4-(α-(4-diethylaminophenyl)-5-hydroxy-2,4-disulfophenyl-methylidene)-2,5-cyclohexadien-1-ylidene]-diethylammonium hydroxide. In practice it appears as a bright blue, water-soluble powder. Manufacturers dissolve it into syrups, glazes, gels and aqueous coatings to deliver an intense blue or — when paired with a yellow colour like E102 or E104 — a vivid green.
Two structural points matter for the rest of this page. First, triarylmethane dyes are chemically distinct from azo dyes (which carry a -N=N- diazo bond) and from most other synthetic food colours. Second, E131 is closely related to E133 (Brilliant Blue FCF), another triarylmethane dye that is far more commonly used in UK food. The two are often discussed together but are not interchangeable on a label — manufacturers must declare each by its own E-number.
Other names you might see for E131 include Food Blue 5, CI 42051, Acid Blue 3 and Bleu Patenté V (the licensed medical brand name in France and the UK).
E131 is a relatively niche ingredient on UK shelves. Where a manufacturer wants a strong blue, E133 (Brilliant Blue FCF) tends to win on colour stability and cost, and most major UK confectionery, fizzy drinks and ice creams that need a blue use E133 rather than E131. That said, E131 does still appear in:
Because UK packaging law requires every food colour to be declared by its E-number or full name in the ingredient list, you can verify E131 by reading the back of the pack. Look for "E131", "Patent Blue V", "colour: Patent Blue V" or "colour (E131)". Many products that carry E131 also carry a yellow companion dye to produce a green hue, so if you spot E131 alongside E102 or E104 in the same ingredient list, that combination is what's making the green.
The same Patent Blue V molecule is licensed in the UK as an injectable lymphatic mapping dye. Surgeons typically inject it interstitially — into the tissue around a tumour — at the start of a sentinel-lymph-node biopsy, most often during breast cancer surgery and melanoma surgery. The dye drains into the lymphatic vessels and stains the first lymph node downstream of the tumour, the so-called sentinel node, a vivid blue. The surgeon can then identify and remove that node for pathology, which guides decisions about further treatment.
This procedure is performed many thousands of times a year across the NHS and private hospitals, which is why the medical literature on Patent Blue V is unusually deep for a substance that doubles as a food colour. The MHRA (the UK Medicines and Healthcare products Regulatory Agency) has issued safety alerts about anaphylaxis risk in surgical use, and modern hospital protocols for sentinel-node biopsy include readiness for anaphylaxis management — clinical staff prepared, drugs and airway equipment to hand — whenever Patent Blue V is given.
That dual use is what makes E131 distinctive. Most synthetic food colours have only ever been studied in the context of food. With E131, decades of surgical practice on millions of patients have produced a far clearer picture of the immunological reactions the molecule can trigger.
Published reviews of sentinel-lymph-node biopsy report Patent Blue V hypersensitivity at roughly 1–2% of injections for mild reactions (transient blue urticaria, itching, mild hypotension) and roughly 0.1–0.5% for severe anaphylactic reactions, with rates varying by review and surgical centre. The mechanism is thought to involve a non-IgE-mediated mast-cell response in many cases, with some IgE-mediated reactions also documented.
Two caveats are critical when carrying these numbers across to food. First, the surgical injection delivers a much higher local concentration of dye than any food serving — milligrams of pure dye injected into tissue versus trace quantities ingested and diluted through the gut. Second, the route is fundamentally different: subcutaneous and intradermal injection bypasses the gastrointestinal barrier, which alters the immune response to a foreign molecule. Food-additive exposure to E131 is orders of magnitude lower in dose and milder in route, and the medical hypersensitivity rates do not directly translate to ingestion.
What the medical data does establish is that E131 is a known immunological agent with a characterised reaction profile, rather than a substance for which hypersensitivity is purely anecdotal. Reactions exist, are reproducible in the clinic, and are taken seriously by regulators and surgeons.
Reports of allergic or hypersensitivity reactions to E131 in food are rare in the published literature, but they exist. Case reports describe urticaria, angioedema and — very rarely — anaphylaxis after ingestion of products containing Patent Blue V. The pattern that recurs in these reports is prior sensitisation: people who have previously reacted to other synthetic dyes, or who have had a documented reaction to Patent Blue V during sentinel-node biopsy, appear over-represented among ingestion-reaction case reports.
E131 is not on the UK list of 14 mandatory allergens (which covers the substances that must be emphasised in ingredient lists, such as milk, egg, soya, gluten and so on). That means manufacturers are not required to flag it the way they flag those allergens, although the colour itself must always be declared by E-number or full name. People with a documented Patent Blue V reaction from prior surgery, or with broad synthetic-dye sensitivity, will want to check ingredient lists; an allergist is the right person to discuss specific avoidance with.
E131 has had a chequered international regulatory history. The United States Food and Drug Administration does not permit Patent Blue V (E131) as a food colour — the US blue dye permitted in food, FD&C Blue No. 1, is the analogue called Brilliant Blue FCF, which UK shoppers know as E133. Australia and New Zealand removed Patent Blue V from their list of permitted food colours in 2018 following a regulatory review. Several other jurisdictions have historically restricted or removed blue dyes including E131. The UK and the EU both permit E131 in food, subject to specific use-level limits in the relevant regulations.
Stating it plainly: a UK pack that lists E131 in the ingredients is fully compliant with UK law. The same product would not be permitted on a US shelf, and would not be permitted in Australia or New Zealand under their current rules. We do not editorialise that gap — different regulators weigh the same evidence differently — but UK shoppers are entitled to know that the international picture is not uniform.
One of the most common confusions about E131 is whether it carries the FSA hyperactivity warning. It does not. The Food Standards Agency advisory wording — "may have an adverse effect on activity and attention in children" — is mandatory only for products containing one or more of six specific colours, all identified by the 2007 Southampton study: E102 (Tartrazine), E104 (Quinoline Yellow), E110 (Sunset Yellow FCF), E122 (Carmoisine), E124 (Ponceau 4R) and E129 (Allura Red AC). All six are predominantly azo or sulphonate dyes — chemically distinct from triarylmethane colours.
E131 is a triarylmethane dye and was not part of the Southampton study cohort. UK packs that contain E131 but none of the Southampton Six are not required to carry the hyperactivity warning, and the FSA has not extended the advisory to E131. People who are avoiding the Southampton Six specifically because of the children's hyperactivity advisory should not assume E131 falls under the same category. People who are avoiding all synthetic dyes as a precautionary choice will treat E131 the same as the rest, but that is a personal preference rather than a regulatory requirement.
The European Food Safety Authority re-evaluated Patent Blue V in 2013 and confirmed an Acceptable Daily Intake of 5 mg/kg body weight per day. The re-evaluation noted the hypersensitivity data from medical use and concluded that food-level exposure remained within the established ADI for typical UK and EU diets. The UK retained EFSA's evaluation post-Brexit, and the Food Standards Agency continues to permit E131 in food at the use-levels set out in retained EU regulation.
Labelling rules in the UK require E131 to be declared by E-number or full name in the ingredient list of any food it appears in. Where the dye is used as part of a colour mix to produce green or other compound shades, each constituent colour must still be listed individually.
E131 is wholly synthetic — there is no animal-derived component in the dye itself. It is generally accepted as suitable for vegan, vegetarian, halal and kosher diets at the additive level. Whether the finished product is suitable depends on the rest of the ingredient list and any certification scheme the manufacturer uses, so a product carrying E131 is not automatically vegan or halal-certified — check the certification mark on the pack rather than reasoning from the colour alone.
Sources we drew on: EFSA Panel on Food Additives and Nutrient Sources, "Scientific Opinion on the re-evaluation of Patent Blue V (E 131) as a food additive", 2013. MHRA safety alerts on Patent Blue V used in surgical lymphatic mapping. Published reviews of sentinel-lymph-node biopsy for breast cancer and melanoma. Food Standards Agency guidance on the Southampton Six and the children's hyperactivity advisory. Food Standards Australia New Zealand 2018 review of Patent Blue V. US FDA list of colour additives permitted for use in food.
Free to log up to 25 foods/day · NutraSafe Pro £3.99/month for AI Coach, allergen warning detail and full reaction history.