Aspartame is now WHO/IARC Group 2B. What that actually means.

In July 2023 the WHO's International Agency for Research on Cancer (IARC) classified aspartame as Group 2B — 'possibly carcinogenic to humans'. The headlines were predictable. The substance is everywhere — diet sodas, sugar-free yogurts, table-top sweeteners — so the news cycle ran hot.

The reality is more interesting and less alarming. IARC and JECFA released their reviews together. JECFA — the joint WHO/FAO expert committee on food additives — left the acceptable daily intake (ADI) unchanged at 40 mg/kg body weight per day. The same week the same agency labelled aspartame 'possibly carcinogenic' and reaffirmed that current intake is well below the level of concern.

What Group 2B means

IARC classifications rate the strength of evidence that something can cause cancer, not how dangerous it is at typical exposure. Group 1 is 'sufficient evidence' (tobacco, alcohol, processed meat). Group 2A is 'probably' (red meat, night-shift work). Group 2B is 'possibly' — limited evidence in humans, limited or sufficient evidence in animals, but neither side conclusive. Aloe vera extract, pickled vegetables and gasoline engine exhaust share the 2B classification.

The aspartame 2B rests primarily on three observational human studies finding modest associations with hepatocellular carcinoma, plus mechanistic and animal data the working group judged 'limited'. None of the human studies had the design strength to establish causation.

What the threshold looks like in practice

JECFA's 40 mg/kg ADI for a 70 kg adult is 2,800 mg per day. A typical 355 ml can of diet soda contains ~180 mg aspartame, so the JECFA / EFSA ADI works out to roughly 14 cans daily, and the higher FDA ADI (50 mg/kg) to roughly 19. Most people are nowhere near either.

PKU is the absolute contraindication

The clear-cut answer is for people with phenylketonuria (PKU): aspartame is hydrolysed to phenylalanine in the gut, and people with PKU cannot metabolise phenylalanine. This is why every aspartame-containing product carries a phenylalanine warning, and PKU is screened in newborns.

How we score it

On the SweetSpot scale aspartame sits at 38 / 100 — Poor. The score reflects three things: a credible Group 2B signal, the presence of cleaner alternatives that didn't exist in 1981, and the practical cost of cleaner taste from monk fruit and allulose at slightly higher prices. Consumers with no PKU and moderate intake are unlikely to be harmed. Consumers with options should choose the options.