Is Aspartame Safe? What the FSA, EFSA, and WHO Actually Say

What is aspartame?

Aspartame is an artificial sweetener approximately 200 times sweeter than sugar. It was discovered accidentally in 1965 by chemist James Schlatter and has been approved for use in food and drink since 1981. It is assigned the E number E951 in the European food additive system.

Chemically, aspartame is made up of two amino acids -- aspartic acid and phenylalanine -- joined by a methanol molecule. When digested, the body breaks aspartame down into these three components, all of which occur naturally in other foods (meat, dairy, fruits, and vegetables contain far higher quantities of all three).

Because aspartame is so much sweeter than sugar, only tiny amounts are needed to achieve the same level of sweetness. This means products sweetened with aspartame contain negligible calories compared to their sugar-sweetened equivalents.

Where you'll find aspartame

Aspartame is one of the most widely used artificial sweeteners in the UK. You'll find it in:

• Diet and sugar-free fizzy drinks -- Diet Coke, Pepsi Max, Coke Zero (in combination with acesulfame K)
• Sugar-free chewing gum -- most major brands including Extra and Orbit
• Tabletop sweeteners -- Canderel, some Hermesetas products
• Sugar-free squash and cordials -- Robinson's, Vimto, many supermarket own-brand
• Low-sugar yoghurts -- various brands use it alongside other sweeteners
• Sugar-free sweets and mints -- Polo Sugar Free, Tic Tac, Smint
• Some medicines -- certain sugar-free versions of cough syrups and chewable tablets

In the UK, any product containing aspartame must list it in the ingredients, either by name or by its E number (E951). Products containing aspartame must also carry the statement: "Contains a source of phenylalanine."

The 2023 WHO assessment: two bodies, two conclusions

In July 2023, two separate WHO-affiliated bodies published assessments of aspartame on the same day. This is where much of the confusion arises, because they appeared to reach different conclusions. In reality, they were answering different questions.

IARC: "Possibly carcinogenic" (Group 2B)

The International Agency for Research on Cancer (IARC) classified aspartame as Group 2B -- "possibly carcinogenic to humans."

IARC's role is to identify whether a substance could cause cancer under any circumstances. It does not assess how much of a substance you would need to consume, or whether normal consumption poses a realistic risk. This is known as hazard identification.

The Group 2B classification was based on "limited evidence" from three observational studies that suggested a possible association between artificially sweetened drinks and liver cancer (hepatocellular carcinoma). IARC noted that the evidence was not conclusive and that confounding factors could not be ruled out.

JECFA: "No reason to change the ADI"

The Joint FAO/WHO Expert Committee on Food Additives (JECFA) simultaneously published its own assessment. JECFA's role is different from IARC's -- it conducts risk assessment, which considers not just whether a substance could be harmful, but at what dose and under what real-world conditions.

JECFA concluded that "the available evidence does not indicate a need to change the existing acceptable daily intake (ADI) of 0-40 mg/kg body weight per day." In other words, aspartame remains safe to consume at the levels people actually consume it.

Dr Francesco Branca, Director of the WHO Department of Nutrition, summarised it clearly: "IARC's finding does not mean that aspartame at commonly consumed levels poses a health risk."

How much aspartame would you actually need to consume?

The ADI of 40mg per kilogram of body weight per day is deliberately conservative -- it includes a 100-fold safety factor built on top of the level at which no adverse effects were observed in animal studies.

For a 70kg adult, the ADI works out to 2,800mg of aspartame per day. To put that in context:

Even a heavy diet drink consumer having 3-4 cans per day would be consuming well under 15% of the ADI. EFSA's 2013 assessment estimated that the highest consumers of aspartame in Europe typically reach 36% of the ADI at most -- and that is the extreme end.

EFSA's 2013 comprehensive safety review

In December 2013, EFSA published what remains the most thorough safety evaluation of aspartame ever conducted. The review took over three years, considered more than 600 datasets from published studies, and was subjected to two rounds of public consultation.

EFSA's key conclusions:

• Aspartame and its breakdown products are safe for human consumption at current exposure levels.
• The ADI of 40mg/kg body weight per day is adequate to protect the general population.
• There is no evidence that aspartame causes cancer in humans at current consumption levels.
• There is no evidence of harm during pregnancy at normal consumption levels (aspartame does not cross the placenta in significant amounts).
• The breakdown products (aspartic acid, phenylalanine, and methanol) are produced in quantities far lower than those occurring naturally from the diet.

EFSA noted that a glass of tomato juice provides approximately six times more methanol than a can of aspartame-sweetened drink. The amounts produced from aspartame are too small to be toxicologically relevant.

The FSA's position

The Food Standards Agency (FSA) -- the UK body responsible for food safety -- bases its position on the EFSA assessment. Following the 2023 IARC/JECFA publications, the FSA stated that it saw no reason to change its advice: aspartame is considered safe for consumption within the established ADI.

The FSA's stance is consistent with regulatory bodies worldwide, including the US FDA, Health Canada, Food Standards Australia New Zealand (FSANZ), and Japan's Ministry of Health.

People who should avoid aspartame: phenylketonuria (PKU)

Beyond PKU, some people report headaches or digestive discomfort after consuming aspartame. The scientific evidence for a causal link is inconsistent -- double-blind trials have generally not replicated self-reported sensitivity. However, if you consistently feel unwell after consuming a particular food or additive, it is reasonable to avoid it regardless of what population-level studies show.

The broader sweetener landscape

Aspartame is one of several artificial and natural sweeteners approved for use in the UK. Here's how they compare:

Many products use blends of sweeteners rather than a single one. Diet Coke, for example, uses aspartame alongside acesulfame K (E950). This is partly for taste (different sweeteners have different flavour profiles and onset/fade characteristics) and partly because combining sweeteners allows lower doses of each individual one.

All approved sweeteners in the UK have undergone safety assessments by EFSA. The choice between them is largely one of personal preference and taste. If you want to know exactly which sweeteners are in a product, scanning the label with a food additive app is the quickest way to find out.

What the evidence does and does not show

It is worth being precise about what the science currently says:

• There is no conclusive evidence that aspartame causes cancer in humans at the levels people actually consume it. The IARC classification reflects limited, inconclusive evidence that warrants further research, not a finding of proven harm.
• Observational studies (where people report what they eat and researchers look for patterns) have shown some associations between artificial sweeteners and health outcomes. But association is not causation -- people who drink large quantities of diet drinks may differ from non-consumers in many ways.
• Randomised controlled trials -- the gold standard for establishing cause and effect -- have not demonstrated that aspartame causes cancer or other serious health effects at normal consumption levels.
• The ADI includes a 100-fold safety margin. It is set at 1/100th of the level at which no adverse effects were observed in animal studies. This is standard practice for food additive safety assessment.

Research is ongoing, and scientific understanding can evolve. The IARC classification is a signal that more high-quality studies would be valuable -- particularly long-term human studies. But based on current evidence, regulatory agencies worldwide continue to consider aspartame safe at normal consumption levels.

Frequently asked questions

Is aspartame safe to consume in the UK?

Yes, according to current regulatory assessments. The FSA considers aspartame safe based on EFSA's comprehensive 2013 review, which set an acceptable daily intake (ADI) of 40mg per kilogram of body weight per day. In 2023, the WHO's JECFA committee reaffirmed this ADI. A 70kg adult would need to consume 12-36 cans of diet fizzy drink daily to exceed this limit.

Why did the WHO classify aspartame as "possibly carcinogenic"?

In July 2023, the WHO's International Agency for Research on Cancer (IARC) classified aspartame as Group 2B -- "possibly carcinogenic to humans." This was based on limited evidence from three observational studies. Group 2B is one of the lower risk categories and includes everyday items like aloe vera, pickled vegetables, and radio-frequency electromagnetic fields (mobile phone signals). It means the evidence warrants monitoring, not that the substance is proven to cause cancer. On the same day, the WHO's JECFA committee reaffirmed that aspartame is safe at current consumption levels.

How many Diet Cokes would you need to drink to exceed the aspartame limit?

A 70kg adult would need to drink approximately 21 cans of Diet Coke per day, every day, to reach the ADI of 40mg/kg body weight. For Pepsi Max, which contains less aspartame per can, the figure is approximately 36 cans. These are conservative limits with a 100-fold safety factor already built in.

Who should avoid aspartame?

People with phenylketonuria (PKU) must avoid aspartame. PKU is a rare inherited condition where the body cannot break down phenylalanine, one of aspartame's components. In the UK, all products containing aspartame must carry the warning "Contains a source of phenylalanine." All newborns are screened for PKU as part of the NHS heel prick test. Beyond PKU, current regulatory evidence does not identify other groups who need to avoid aspartame.

Is aspartame worse than other artificial sweeteners?

All artificial sweeteners approved for use in the UK -- including aspartame (E951), sucralose (E955), acesulfame K (E950), and stevia (E960) -- have been assessed and deemed safe at their respective ADI levels by EFSA. No single approved sweetener is considered more or less safe than another when consumed within guidelines. Each has a different taste profile, which is why many products use blends. For a broader look at food additives, see our guide to whether E numbers are bad for you.

Related articles

• Are E Numbers Bad for You? A Science-Based Guide
• What Are E Numbers? The Complete UK Guide
• Aspartame (E951) -- Full Safety Profile
• Artificial Sweeteners: Side Effects and What the Research Says
• Food Additive Scanner -- Check Any Product Instantly

Last reviewed and updated: 7 February 2026