Patent Blue V (E131): The Synthetic Blue Dye and the Skin
Patent Blue V (E131) is a synthetic blue colour in sweets, drinks and some medicines. Food reactions are uncommon, but they happen in sensitive people. What it is, who reacts, and where it hides.
What it is
Patent Blue V is a synthetic blue colour, listed as E131 and also known as Acid Blue 3. It belongs to the triarylmethane family of dyes, the same group as Brilliant Blue, and it is not an azo dye. That distinction matters, because much of the concern around food colours and skin reactions centres on the azo dyes, while Patent Blue V sits in a different chemical class. It is often blended with a yellow colour to produce green shades. European food-safety regulators re-evaluated it and concluded it is acceptable within set intake limits, as set out in the EFSA opinion on Patent Blue V.
Who reacts, and how it shows on the skin
It is worth being measured here. From eating it as a food colour, skin reactions are uncommon, but they are reported in sensitive people: hives, itching and flushing, usually as a pseudo-allergic response rather than a true allergy. People with chronic urticaria who seem to react to a range of additives, and those with aspirin sensitivity or asthma, are the groups most often mentioned. Separately, the same dye injected in hospital procedures has caused more serious allergic reactions, but that is at far higher doses than anything in food and should not be read across to a sweet or a drink.
Where it hides
Blue and green sweets, ice pops and slushies, some soft and sports drinks, blue mixers, cake decorations and icing, and non-food items such as some medicines and Scotch whisky colourings. On a label, look for Patent Blue V, E131 or Acid Blue 3, and remember that a green product often contains a blue plus a yellow, and the yellow may be an azo dye you also want to screen for.
What to do if you think you react
Keep a food-and-skin diary and note brightly coloured drinks and sweets in particular, since colours are easy to overlook. If a pattern shows up, check green products too because of the blue-plus-yellow blend. The NHS guidance on food allergy and intolerance is a sensible starting point, and you should see a GP or allergist if reactions are frequent or severe rather than cutting out lots of foods on your own.
Check it against your own list
A free database like Open Food Facts lets you look up a packaged product and see its additives and colours listed, a useful broad read, though the information is the same for everyone rather than tuned to your skin. To check a product against the specific things that make your skin react, a personal-list app like ClearaScan lets you save your triggers once and scan any product, food, medication or cosmetic, against your guard list, flagging only yours. Its Reaction Journal lets you tie a flare back to the product that caused it, a shared Care Circle lets family scan for you, and a Trusted Products list keeps what you have cleared. It is currently in early access. (Disclosure: our editor co-founded ClearaScan, and we are not paid to mention the others.)
The Sensitive Skin Ingredient Decoder
A free one-page guide to reading a skincare label fast: the irritants to watch, the fragrance loopholes, and what "fragrance-free" and "hypoallergenic" really mean.
Common questions
Is Patent Blue V an azo dye?
No. It is a triarylmethane dye, the same broad family as Brilliant Blue, and chemically different from azo colours like tartrazine and Sunset Yellow. Much of the worry about food colours focuses on the azo dyes specifically.
Is E131 banned?
It is permitted within set limits in the UK and EU. It is not approved as a food colour in the United States, which is one reason you see it less on US shelves. Regulators have reviewed it and set an acceptable daily intake.
Why is Patent Blue V used in hospitals?
The same dye is injected during some operations to map lymph nodes. That medical use, at much higher doses than food, is where most serious allergic reactions have been recorded, which is different from eating a trace in a sweet.
Should I avoid it if I have chronic hives?
Some people with chronic urticaria find that several additives together seem to contribute to flares. If you suspect colours, a food-and-skin diary is the practical way to test the idea before cutting things out.