Aspartame
A man-made low-calorie sweetener, roughly 200 times sweeter than sugar, used in diet drinks and sugar-free foods. The WHO's cancer agency lists it as a possible cause of cancer.
The WHO cancer agency rates it a possible cause of liver cancer, based on limited human evidence. People with the inherited condition phenylketonuria must avoid it because their bodies cannot process one of its breakdown products.
What is it?
A synthetic sweetener made by joining two amino acids, aspartic acid and phenylalanine, with a small amount of methanol attached. It carries almost no calories because so little is needed for sweetness.
What does it do?
It tastes about 200 times sweeter than sugar, so tiny amounts replace sugar without the calories. In the body it breaks down into phenylalanine, aspartic acid and methanol, the same substances found naturally in many foods.
Where you will see it
Diet and zero-sugar fizzy drinks, sugar-free squash and chewing gum, low-calorie yoghurts and desserts, and table-top sweeteners. On a label it reads as 'aspartame' or 'E951', and packs must also state 'contains a source of phenylalanine'.
What the science says
Cancer hazard classification
In 2023 the WHO's cancer research agency reviewed aspartame for the first time and placed it in its 'possibly carcinogenic to humans' category, citing limited human evidence linking it to liver cancer. The agency stressed the evidence had limitations, including how studies estimated intake. This is a hazard rating, meaning it flags a potential, not a measure of real-world risk at typical intakes.
IARC classified aspartame as Group 2B, possibly carcinogenic to humans, on the basis of limited evidence for hepatocellular carcinoma (liver cancer).
The companion JECFA review found no sufficient reason to change the acceptable daily intake of 0 to 40mg per kg body weight.
Human dietary studies
A large French cohort following over 100,000 adults reported that higher consumers of artificial sweeteners, aspartame among them, had a modestly higher rate of cancer overall and of breast and obesity-related cancers. This is an observational association, not proof that aspartame causes cancer, and the authors noted possible bias, confounding and reverse causality.
Higher aspartame intake was associated with a slightly increased overall cancer risk (hazard ratio 1.15) and higher breast cancer risk (hazard ratio 1.22) in the NutriNet-Sante cohort of 102,865 adults.
Regulatory intake limit
EU and UK regulators set an acceptable daily intake of 40mg per kg of body weight, based on chronic-toxicity studies in animals. For an adult this is a large amount, roughly equivalent to many cans of diet drink a day, so typical consumers stay well below it.
EFSA's 2013 re-evaluation of aspartame established an acceptable daily intake of 40mg per kg body weight per day.
Phenylketonuria
Aspartame breaks down to release phenylalanine. People born with phenylketonuria (PKU) cannot break phenylalanine down properly, so it builds up and can harm the brain. For this reason every aspartame-containing product must warn that it is a source of phenylalanine.
In the gut aspartame is fully broken down to phenylalanine, aspartic acid and methanol; the released phenylalanine poses a risk to people with phenylketonuria.
Where it stands with the regulators
Who should be careful
Anyone with phenylketonuria (PKU) must avoid it, including children identified by the newborn heel-prick test. Look for 'aspartame', 'E951' or 'contains a source of phenylalanine' on the label.
The honest read
Two official 2023 verdicts pull in different directions and both are real: the WHO's cancer agency rated aspartame a possible carcinogen on limited human evidence, while the WHO's food-additives committee kept the intake limit unchanged. Observational studies link higher intake to a small rise in cancer rates, but association is not proof of cause, and the heaviest study weight rests on animal data and intake limits rather than settled human harm. The disagreement is genuine and the research is ongoing.
Related additives
Common questions
Is E951 banned in the UK?
No. Aspartame is approved for use as a sweetener in Great Britain and the EU under assimilated Regulation 1333/2008, with conditions of use and mandatory phenylalanine labelling.
Does E951 cause cancer?
In 2023 the WHO's cancer agency (IARC) classified aspartame as 'possibly carcinogenic to humans' (Group 2B) on limited human evidence pointing to liver cancer. That is a hazard flag, not proof of cancer at normal intakes. The WHO's food committee left the intake limit unchanged. The science is unsettled.
What foods contain E951?
Diet and zero-sugar fizzy drinks, sugar-free chewing gum, low-calorie yoghurts and desserts, sugar-free squash and table-top sweeteners. It appears on labels as 'aspartame' or 'E951' alongside a phenylalanine warning.
Is E951 vegan?
Aspartame is synthesised from amino acids and is not derived from animals, so it is generally suitable for vegans. Always check the finished product, as the food it sits in may not be.
Sources
- Aspartame hazard and risk assessment results released, WHO/IARC/JECFA (2023)
- Scientific Opinion on the re-evaluation of aspartame (E 951) as a food additive, EFSA Journal (2013)
- Debras et al., Artificial sweeteners and cancer risk: NutriNet-Sante cohort, PLOS Medicine (2022)
- Approved additives and E numbers, UK Food Standards Agency
- Regulation (EU) No 1169/2011 on food information to consumers
Aaron Keen is the founder of NutraSafe. He researches and writes every additive entry himself, from the primary sources. About the research →
This is a guide, not medical advice. If an additive affects you, speak to your GP or a dietitian.
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