Red 40 (also called Allura Red AC) is a synthetic food dye that gives products a bright red or orange-red color. It has no flavor, no nutritional value, and no function beyond appearance. But when people search “what does Red 40 do,” they’re usually asking about what it does inside the body, particularly in children. The short answer: in some people, it can trigger histamine release and may subtly affect attention and behavior in kids, though the effects are small and not universal.
Where You’ll Find It
Red 40 is one of the most widely used color additives in the food supply. It shows up in places you’d expect and plenty you wouldn’t. Obvious sources include candy, fruit snacks, sports drinks, soda, and flavored gelatin. Less obvious ones include flavored yogurt, strawberry milk, breakfast cereals, protein powders, puddings, ice cream, chips, cakes, and chewing gum. It’s also used in cosmetics and pharmaceuticals, meaning everything from lipstick to liquid cold medicine can contain it.
On ingredient labels in the U.S., it appears as “Red 40,” “FD&C Red No. 40,” or “Allura Red AC.” In Europe, it’s listed as E129.
How It Can Affect the Body
Red 40 is not a true allergen. Children’s Hospital of Philadelphia notes there are no confirmed reports of allergic reactions to synthetic red dyes like Red 40. (The natural red dye carmine, made from insects, is the only red colorant known to cause genuine allergic reactions, and even those are rare.)
That said, Red 40 can trigger a sensitivity response in some people. In those who are sensitive, the dye prompts the body to release histamine, the same chemical involved in seasonal allergy symptoms. This can lead to headaches, hives, skin irritation, sneezing, watery eyes, and in some cases worsening of asthma. This isn’t the same mechanism as a food allergy. It’s more like an intolerance: your immune system isn’t producing antibodies against the dye, but your body still reacts.
The Link to Hyperactivity in Children
The most studied concern about Red 40 is whether it affects children’s behavior, specifically attention and hyperactivity. The evidence points to a real but small effect, concentrated in kids who are already predisposed.
A 2004 meta-analysis pooling 15 clinical trials with 219 subjects found a statistically significant effect of artificial food colors on children’s behavior, with an effect size of 0.28. To put that in context, that’s a modest effect, smaller than what you’d typically see from caffeine or a poor night’s sleep, but large enough to be measurable. Interestingly, the effect was most apparent when parents rated their children’s behavior (effect size 0.44) and essentially invisible when teachers or clinicians did the rating.
A landmark 2007 study funded by the UK Food Standards Agency tested two different mixtures of food dyes (both containing Red 40 along with other artificial colors and the preservative sodium benzoate) on three-year-olds and eight- to nine-year-olds. Both age groups showed increased hyperactivity with at least one of the mixtures, though results varied by age group and which mixture was tested. Because the mixtures combined multiple dyes with a preservative, it’s difficult to isolate Red 40’s individual contribution.
Genetics appear to play a role. A 2010 follow-up analysis found that three-year-olds with certain variations in genes related to histamine processing showed a stronger behavioral response to food dyes. Children without those genetic variations were less affected. This helps explain why some kids seem visibly wired after eating brightly colored snacks while others show no change at all.
How Different Countries Handle It
Red 40 remains legal in both the United States and Europe, but the regulatory approaches differ sharply. The FDA considers it safe at current levels of consumption and requires no warning labels. The European Union takes a precautionary stance. Since July 2010, any food or drink sold in the EU containing Red 40 (or five other synthetic dyes) must carry a label stating the dye “may have an adverse effect on activity and attention in children.”
This doesn’t mean European regulators concluded the dye is dangerous. It means they decided the evidence was strong enough that parents deserved a heads-up. Many European food manufacturers reformulated their products with natural colorants simply to avoid printing that warning.
What This Means in Practice
If you or your child have no noticeable reaction to foods containing Red 40, the existing evidence doesn’t suggest a major health risk. The behavioral effects shown in studies are subtle and population-level, meaning they’re real on average but not dramatic for most individuals.
If your child already struggles with attention or hyperactivity, a trial period of removing artificial dyes from their diet is a low-risk experiment. Some children are clearly more sensitive than others, and the genetic research supports the idea that this isn’t just parental imagination. You won’t see results on a blood test; the only way to know is to remove the dyes for a few weeks and observe.
For people who experience headaches, hives, or other physical symptoms after consuming Red 40, avoidance is straightforward but requires label reading. The dye hides in products that aren’t even red, including some chocolate cake mixes, barbecue sauces, and orange-colored snacks, where it’s blended with yellow dyes to achieve a specific shade.