In July 2023 the World Health Organization’s cancer research agency, IARC, classified aspartame as “possibly carcinogenic to humans,” its Group 2B. That rating describes how strong the evidence of a possible hazard is, not how likely a diet soda is to give you cancer. In the same announcement, a separate WHO and FAO expert committee kept the long-standing acceptable daily intake for aspartame unchanged. The U.S. Food and Drug Administration, for its part, said it disagreed with the IARC classification. For most people the useful takeaway is about amounts. For one group, people with the inherited condition PKU, the advice is different and much firmer.
Aspartame is a two-amino-acid sweetener
The federal food additive rule for aspartame, 21 CFR 172.804, defines it chemically as 1-methyl N-L-α-aspartyl-L-phenylalanine. In plain terms, it is two amino acids, aspartic acid and phenylalanine, joined like a tiny two-unit peptide with a small methyl group attached. That structure is why the same rule requires a phenylalanine warning on labels.
The WHO joint news release lists where aspartame turns up: diet drinks, chewing gum, gelatin, ice cream, yogurt and other dairy products, breakfast cereal, toothpaste, and medicines such as cough drops and chewable vitamins. Some of those are easy to forget when you are adding up what you consume.
Hazard versus risk: what the two WHO groups said
The two groups answered different questions. IARC does hazard identification: it asks whether something could cause cancer under some circumstances. According to the WHO release, IARC found limited evidence in humans, specifically for a type of liver cancer called hepatocellular carcinoma, plus limited evidence in experimental animals and limited evidence about possible mechanisms. Group 2B is the third-highest of IARC’s four levels, and the release states plainly that IARC classifications “do not reflect the risk of developing cancer at a given exposure level.”
The Joint FAO/WHO Expert Committee on Food Additives (JECFA) does risk assessment: how likely harm is at real-world amounts. It found no sufficient reason to change the acceptable daily intake of 0 to 40 milligrams per kilogram of body weight. Its own worked example: if a can of diet soft drink contains 200 or 300 mg of aspartame, a 70 kg adult would need to drink more than 9 to 14 cans a day to exceed that limit, assuming no other sources. JECFA called the evidence linking aspartame to cancer in people “not convincing” and asked for better, longer studies.
The FDA’s response went further. It said the IARC label “does not mean that aspartame is actually linked to cancer,” that agency scientists had found significant shortcomings in the studies IARC relied on, and that JECFA had not raised safety concerns at current levels of use.
What the human studies show so far
One of the larger human studies in the debate is the French NutriNet-Santé cohort, published in PLOS Medicine in 2022. It followed 102,865 adults for a median of 7.8 years using repeated food records. People in the higher aspartame-intake group had about a 15% higher rate of cancer overall than non-consumers (hazard ratio 1.15). The authors list the limits themselves: possible selection bias, residual confounding and reverse causality. A study like this can flag a pattern worth checking; it cannot show that the sweetener caused the cancers, which is part of why the evidence is graded “limited.”
Who should avoid aspartame entirely
MedlinePlus explains that phenylketonuria (PKU) is an inherited condition in which the body cannot process phenylalanine, and that high levels can damage the brain. Babies born in U.S. hospitals are screened for it. Because aspartame contains phenylalanine, the federal rule requires any food containing it to carry the statement “PHENYLKETONURICS: CONTAINS PHENYLALANINE,” printed prominently in bold type.
If you or your child has PKU, that line is the one to look for, including on chewable vitamins, cough drops and sugar-free gum, and a pharmacist can check medicines whose labels are hard to read.
For everyone else, the practical reading is this: the 2023 reviews did not find that ordinary amounts are unsafe, and they did agree that the research questions are still open. If you drink several diet sodas a day and would rather cut back while that research continues, that is a reasonable personal choice. Just compare what you replace them with, rather than assuming any swap is automatically healthier.
By The Everyday Brief · AI-assisted editorial. Drafted with AI research assistance; claims checked against the linked sources (fetched 2026-10-05 PT). Not medical advice.
Sources
- World Health Organization: Aspartame hazard and risk assessment results released (joint IARC/WHO/FAO news release, 14 July 2023)
- U.S. Food and Drug Administration: Aspartame and Other Sweeteners in Food
- Electronic Code of Federal Regulations: 21 CFR 172.804, Aspartame
- Debras C, et al. Artificial sweeteners and cancer risk: Results from the NutriNet-Santé population-based cohort study. PLOS Medicine, 2022.
- MedlinePlus (U.S. National Library of Medicine): Phenylketonuria