Studies raise questions; harm in humans has not been established.
In July 2023 the International Agency for Research on Cancer classified aspartame as possibly carcinogenic to humans (Group 2B), citing LIMITED evidence for hepatocellular carcinoma (a liver cancer) in humans, limited evidence in animals, and limited mechanistic evidence.
IARC explicitly noted that chance, bias, or confounding could not be ruled out in the human studies. Two things make this weaker than headlines suggested. First, Group 2B is IARC's second-lowest evidence tier and reflects hazard identification — whether a substance could cause cancer under some conditions — not risk at normal dietary exposure. Second, on the same day, the WHO-FAO Joint Expert Committee on Food Additives (JECFA), which performs the quantitative risk assessment, reviewed the same evidence and reaffirmed the existing acceptable daily intake of 40 mg/kg body weight unchanged, concluding aspartame is safe within that limit. For scale, a 70 kg adult would need more than 9-14 cans of diet soda daily to exceed it. FDA disagreed with IARC's classification outright and maintains a US ADI of 50 mg/kg. This is therefore a genuine disagreement between official bodies rather than a settled finding in either direction. Separately and unambiguously: people with phenylketonuria (PKU) must avoid aspartame, as it is a source of phenylalanine — this is why products carry a PKU warning.