Red No. 3 (erythrosine) stands out as the red dye with the strongest evidence of harm, having been linked to thyroid tumors in animal studies and recently revoked for use in food by the FDA. Red No. 40 (Allura Red AC), the most widely used red dye in the American food supply, carries a different set of concerns centered on children’s behavior and gut inflammation. Beyond those two, several other synthetic and even natural red colorants come with their own risks worth knowing about.
Red No. 3 and the Thyroid Problem
Red No. 3, also known as erythrosine, is the dye that finally pushed regulators to act. It gives a cherry-red color to candies, cake decorations, and certain medications. The core worry is thyroid cancer. In a long-term feeding study, male rats given a high dose of erythrosine developed thyroid follicular cell tumors at significantly elevated rates, with thyroid gland weights roughly doubling compared to controls.1Food and Chemical Toxicology. Lifetime toxicity/carcinogenicity study of FD & C Red No. 3 (erythrosine) in rats A separate study found that Red No. 3 promoted thyroid tumor development in rats that had already been exposed to a known carcinogen and had partial thyroid removal, pushing tumor incidence above 90 percent in that group.2PubMed Central. The promoting effects of food dyes, erythrosine (Red 3) and rose bengal B (Red 105), on thyroid tumors in partially thyroidectomized N-bis(2-hydroxypropyl)-nitrosamine-treated rats
The mechanism behind this is not a mystery. Erythrosine’s molecular structure contains four iodine atoms, and its shape is similar enough to thyroid hormones that it can interfere with the way the body handles them. X-ray crystallography has confirmed that the dye shares structural features with T4, the main thyroid hormone, allowing it to compete for hormone-binding sites on proteins.3PubMed. Conformational analysis of erythrosine B (FD&C Red No. 3) and its comparison with thyroid hormone structures In rats, erythrosine was shown to reduce the liver’s ability to convert T4 into T3 (the more active thyroid hormone), which then triggers the pituitary gland to pump out more thyroid-stimulating hormone. That chronic overstimulation of the thyroid is believed to drive the tumor growth seen in rodent studies.4Toxicology and Applied Pharmacology. Effects of oral erythrosine (2′,4′,5′,7′-tetraiodofluorescein) on the pituitary-thyroid axis in rats
Whether this same chain of events plays out in humans at typical dietary doses remains debated. The doses used in rodent studies were far higher than what people consume. Still, the FDA invoked the Delaney Clause, which prohibits any food additive shown to cause cancer in animals, and formally revoked authorization for Red No. 3 in food. The ban takes full effect in 2027 for foods and 2028 for ingested drugs, giving manufacturers time to reformulate.
Red No. 40 and Children’s Behavior
Red No. 40 (Allura Red AC) is a different story. It is the most widely consumed artificial food dye in the United States, showing up in everything from sports drinks and fruit snacks to breakfast cereals and flavored yogurts. The primary concern here is not cancer but behavioral effects in children.
A review of the human and animal evidence found that of 25 challenge studies examining artificial food colors and children’s behavior, the majority identified some evidence of a link, with over half reaching statistical significance.5PubMed Central. Potential impacts of synthetic food dyes on activity and attention in children: a review of the human and animal evidence These behavioral effects, including increased hyperactivity and decreased attention, are not limited to children who have been diagnosed with ADHD. Research suggests artificial food colors affect children more broadly, making this more of a general public health issue than an ADHD-specific one.6PubMed Central. Artificial food colors and attention-deficit/hyperactivity symptoms: conclusions to dye for
The effect size is generally described as small but real. To put that in perspective, a small average effect across millions of children eating dye-laden foods daily adds up to a meaningful population-level impact, even if no individual child’s life is dramatically altered. This is the logic behind why the European Union requires warning labels on foods containing certain synthetic dyes, including Red No. 40, stating that the product “may have an adverse effect on activity and attention in children.”
Red No. 40 and Gut Inflammation
A newer and increasingly concerning line of research links Red No. 40 to gut health problems. In mice, chronic exposure to Allura Red AC promoted susceptibility to experimental colitis. The dye elevated serotonin levels in the colon and disrupted the intestinal barrier, triggering low-grade inflammation even without an additional chemical insult to the gut.7Nature Communications. Chronic exposure to synthetic food colorant Allura Red AC promotes susceptibility to experimental colitis via intestinal serotonin in mice The gut microbiome also shifted, with distinct clustering patterns between exposed and unexposed animals.
A separate mouse study found that Red No. 40, particularly when combined with a high-fat diet, reduced beneficial gut bacteria and increased harmful ones. Protective species dropped while inflammation-associated groups expanded.8PubMed Central. The synthetic food dye, Red 40, causes DNA damage, causes colonic inflammation, and impacts the microbiome in mice That same study reported evidence of DNA damage in the colon of exposed mice.
Research published in Cell Metabolism added nuance: Red No. 40 alone did not cause colitis in genetically susceptible mice, but it did trigger severe relapsing-remitting colitis when the animals’ immune systems were primed. Interestingly, the colitis depended on the presence of gut bacteria even though the dye did not cause major shifts in overall microbial composition.9Cell Metabolism. Food colorants are dietary risk factors for inflammatory bowel disease In other words, Red No. 40 may not wreck a healthy gut on its own, but it could tip the scales in people who already have inflammation or a genetic predisposition to inflammatory bowel disease.
All of this is animal data, and mouse guts are not human guts. But the consistency across multiple labs and study designs is hard to dismiss. For people already managing conditions like Crohn’s disease or ulcerative colitis, these findings suggest some caution around heavily dyed processed foods is reasonable, even before definitive human trials arrive.
What Happens When Red No. 40 Breaks Down
One underappreciated concern with Red No. 40 is what happens after you eat it. Your gut bacteria metabolize the dye, and the breakdown products may matter as much as the dye itself. Laboratory work using common gut bacteria (Enterobacteria) to degrade Red No. 40 found evidence that the process generates compounds similar to 1-naphthol and aromatic amines.10Journal of Pure and Applied Microbiology. Characterization of the Degradation Products from the Red Dye 40 by Enterobacteria Aromatic amines are a chemical class with known links to cancer in other contexts, so the fact that gut bacteria can produce them from a food dye raises flags, even if the amounts are small and the pathway from petri dish to human disease is long.
This is part of a broader worry about azo dyes, the chemical family that Red No. 40 belongs to. Azo dyes share a nitrogen-nitrogen double bond that gut bacteria can cleave, and the fragments produced depend on which specific dye is being broken down. Red No. 40 is far from the only azo dye in the food supply, and there is growing interest in whether the cumulative load of these breakdown products from multiple dyes eaten together could matter more than any single dye consumed in isolation.
How Much Red Dye Are Children Actually Eating
The question of real-world exposure is critical, because a dye that causes problems at extreme doses but barely shows up in actual diets is a different concern than one children consume at meaningful levels every day. An analysis of dye levels in foods commonly eaten by children found that Red No. 40 was the dominant red dye across most product categories. For an average-weight child aged six to ten, a single serving of a dyed drink could deliver up to about 7 to 9 percent of the U.S. acceptable daily intake (ADI) for Red No. 40 alone.11Journal of Food Composition and Analysis. Levels of FD&C certified food dyes in foods commonly consumed by children
That might sound reassuringly low, but the picture changes once you consider that kids often eat multiple dyed products in a single day. When all azo dyes (Red No. 40 plus yellows and oranges) were considered together, the combined exposure from drinks alone reached 10 to 22 percent of the ADI per serving.11Journal of Food Composition and Analysis. Levels of FD&C certified food dyes in foods commonly consumed by children A child who has a colored cereal for breakfast, a fruit snack at school, and a sports drink after soccer practice could easily approach or exceed the ADI on a busy day. And the ADI itself is based on older safety data that did not account for the behavioral and gut-health findings that have emerged more recently.
Other Synthetic Reds You Might Encounter
Red No. 3 and Red No. 40 get the most attention, but they are not the only synthetic red colorants. Depending on where you live or what imported products you buy, you could run into several others.
Ponceau 4R (also called Red No. 7 in some systems, or E124 in Europe) is widely used outside the United States but is not approved for food use in the U.S. It is one of the dyes that triggers mandatory warning labels in the EU. A study of chewing gum products sold through Indian online marketplaces detected Ponceau 4R, erythrosine, and azorubine in products that would violate U.S. regulations if sold there.12PubMed Central. Color additives in chewing gum products sold through Indian online marketplaces: variances and compliance with food safety norms in India, the EU, and the USA
Citrus Red 2 is a dye used exclusively to color the skin of some Florida oranges. It is not added to anything you eat directly, but a toxicology review flagged safety concerns about it, noting the limited use keeps exposure low.13PubMed. Toxicology of food dyes Amaranth (Red No. 2) was banned in the United States in the 1970s over cancer concerns but remains legal in some other countries. The patchwork of global approvals and bans can be disorienting if you are shopping for imported foods or traveling.
Why the U.S. and Europe Handle Red Dyes Differently
A comparison of EU and U.S. food color regulations found that while both systems rely on risk assessment, the details and implementation diverge enough to affect what ends up on store shelves. Producers who want to sell the same product in both markets often have to reformulate because a dye permitted in one jurisdiction may be restricted or banned in the other.14PubMed. Comparison of food colour regulations in the EU and the US: a review of current provisions
The EU took action on behavioral concerns earlier. After a widely cited 2007 study (the “Southampton study”) found that mixtures of food dyes and the preservative sodium benzoate increased hyperactivity in children, the EU required warning labels on products containing six specific dyes, including Red No. 40 and a variant of Red No. 3 used there. Many European food manufacturers simply switched to natural colorants rather than carry a warning label, which is why a British Kit-Kat might get its color from beetroot extract while an American one uses Red No. 40.
The U.S., by contrast, allowed both Red No. 3 and Red No. 40 to continue in food for decades after the initial concerns surfaced. The FDA’s position was historically that the evidence of behavioral effects was inconsistent and that existing exposure levels were safe. The recent ban on Red No. 3 marks a shift, but it was driven specifically by the animal cancer evidence under the Delaney Clause rather than the behavioral data. Red No. 40 remains fully approved in the U.S. with no warning label requirement.
Natural Red Dyes Are Not Automatically Safe
When manufacturers reformulate away from synthetic dyes, they typically reach for natural alternatives: carmine (from cochineal insects), beetroot extract (betanin), or lycopene (from tomatoes). These carry a “clean label” appeal, but they come with their own concerns.
Carmine, one of the most widely used natural red colorants, is derived from the bodies of cochineal scale insects. It produces a vivid red and is remarkably stable, but it can trigger genuine allergic reactions. The primary allergen is thought to be a contaminating 38-kDa protein present in cochineal extract. Cases of immediate allergic reactions, including anaphylaxis, have been documented.15PubMed. Cochineal dye-induced immediate allergy: Review of Japanese cases and proposed new diagnostic chart These reactions are rare in the general population, but they are not trivial for the people who experience them. The FDA requires that carmine and cochineal extract be declared on ingredient labels, partly in response to these allergy reports.
Beetroot-derived betalains are another popular substitute. Beetroot juice can produce attractive red tones in plant-based meat analogs and other products, and it provides some antioxidant activity alongside its color.16Food Bioscience. Beetroot juices as colorant in plant-based minced meat analogues: Color, betalain composition and antioxidant activity as affected by juice type The tradeoff is stability. Betanin degrades faster than synthetic dyes when exposed to light or heat, which is why some naturally colored products fade in appearance over their shelf life or require opaque packaging.
Lycopene, sourced from tomatoes, and anthocyanins from fruits like grapes and elderberries are other options you will see. None of these natural pigments carry the same toxicological baggage as Red No. 3, but they can be more expensive, less stable, and occasionally allergenic. The takeaway is not that natural is better or worse across the board, but that swapping one colorant for another involves trade-offs that go beyond “synthetic equals bad.”
Pseudoallergic Reactions and Overstated Fears
Among the broader public, there is a widespread belief that food dyes cause a significant number of allergic reactions, including hives and asthma attacks. The reality is more restrained. While controlled challenge studies have confirmed that some individuals react to synthetic dyes with urticaria (hives) or worsened asthma, most people who believe they are sensitive to food dyes test negative when given the dyes under blinded, controlled conditions.13PubMed. Toxicology of food dyes The reactions that do occur are typically “pseudoallergic,” meaning they look like allergic reactions but do not involve the classic IgE-mediated immune pathway. They are real but much less common than anecdotal reports suggest.
This does not mean you should dismiss someone who consistently gets hives after eating brightly colored candy. Individual sensitivity exists. But the idea that food dyes are a common allergen on par with peanuts or shellfish is not supported by the clinical evidence. The bigger population-level concerns remain the behavioral effects in children and the emerging gut-health data.
Reading Labels and Reducing Exposure
If you want to reduce your family’s exposure to synthetic red dyes, the practical steps are straightforward. In the U.S., food dyes must be listed by name on ingredient labels, so look for “Red 40,” “Red 3,” “Allura Red,” or “Erythrosine.” You will also see FD&C numbers (FD&C Red No. 40, for example). In Europe, look for E-numbers: E129 for Red No. 40, E127 for Red No. 3, E124 for Ponceau 4R.
The foods most likely to carry high dye loads are the ones that look the most vibrantly colored: fruit-flavored drinks, candy, frosted baked goods, flavored cereals, and certain snack bars marketed to children. Medications and dietary supplements can also contain Red No. 3 or Red No. 40, and the reformulation timelines for drugs lag behind food. If you take a bright red or pink over-the-counter medication and want to avoid these dyes, check the inactive ingredients list or ask your pharmacist about alternatives.
Products labeled “no artificial colors” have made the switch already, typically to beet juice, paprika extract, or fruit and vegetable juice concentrates for red hues. Store-brand and budget products tend to use synthetic dyes more often than premium brands, though this is not universal. Some fast-food chains and major candy manufacturers have voluntarily removed synthetic dyes from products sold in Europe while keeping them in the U.S. versions of the same items, so the same product can contain different colorants depending on which country you buy it in.14PubMed. Comparison of food colour regulations in the EU and the US: a review of current provisions
The Mixture Question Nobody Has Answered Well
Almost all safety testing of food dyes has been done one dye at a time. In reality, a single snack can contain three or four different synthetic dyes, and a child eating a typical processed-food diet could ingest half a dozen across a single day. The question of whether dyes interact, amplifying each other’s effects, is one of the biggest open gaps in the research.
Some early challenge studies used mixtures of dyes (the Southampton study being the most famous), and those mixtures produced effects on behavior. But it is difficult to disentangle whether the mixture itself was more harmful than the sum of its parts or whether combining dyes simply meant a higher total dose. Animal studies have occasionally tested pairs or trios of dyes and found synergistic effects on markers like oxidative stress, but the data are too thin to draw firm conclusions.
Regulatory agencies set acceptable daily intakes for each dye individually. There is no standard framework for evaluating cumulative exposure to multiple dyes in a single diet, which critics argue is a significant blind spot. Until mixture toxicology catches up, the practical advice remains straightforward: eating fewer heavily processed, vibrantly colored foods reduces your total dye load regardless of which specific dyes are in the mix.