WHO Declares Aspartame a Possible Carcinogen

The International Agency for Research on Cancer (IARC) announced today that it has officially classified the artificial sweetener aspartame as a possible carcinogen.

What is aspartame?

Aspartame is one of the most commonly used artificial sweeteners in the world.

It has been on the market for decades. The Calorie Control Council, an international association that represents the low- and reduced-calorie food and beverage industry, says aspartame is found in about 6,000 products globally.

Aspartame can show up in places you might not expect, like in toothpastes or medications, but it’s more often seen on the labels of products marketed as “diet” or “sugar free.”

Sodas like Diet Coke, Coke Zero and Pepsi Zero Sugar have aspartame, as do many low-calorie coffee sweeteners like Equal and NutraSweet, and some juices. In food, aspartame is often in no-sugar salad dressing, low-calorie ice cream, gelatins and puddings like Jell-O Sugar Free Instant Pudding. It’s also in sugar-free gum like Extra.

Although WHO said in a separate decision in May that people shouldn’t rely on non-sugar sweeteners for weight control, aspartame is often used in “diet” drinks because it has fewer calories than regular sugar. Compared with regular table sugar, aspartame is also about 200 times sweeter, so products don’t need as much of it.

If you put a package of Equal in your coffee, it would have about the same sweetness as 2 teaspoons of regular sugar. The packet of Equal has 4 calories, but two teaspoons of sugar have 32 calories.

Initial confusion about aspartame

There’s been some confusion about aspartame from the beginning.

In 1974, the FDA approved the use of aspartame in some foods and beverages, but the decision was suspended for a few years because of some contradictory studies, objections to its approval and questions about the initial studies themselves. Some scientists were concerned when an early animal study showed that aspartame may have caused brain tumors in rats.

It wasn’t until 1981, after a thorough investigation, that the FDA finally allowed the marketing of aspartame in dry foods. That year, the FAO/WHO Expert Committee on Food Additives determined the parameters for an acceptable daily intake.

Aspartame guidelines

WHO’s guidelines haven’t changed since 1981: a daily maximum of 40 milligrams of aspartame per kilogram of body weight. The US recommendations are slightly more generous; in 1983, the FDA set the guideline at 50 milligrams per kilogram of body weight.


Although this means aspartame might cause cancer in humans, the Group 2B classification from the IARC means the evidence is “limited.” A summary of the working group’s evaluation, also published today in Lancet Oncology, explained that the classification was based on data from three studies assessing the link between aspartame intake and primary liver cancer.

Using that evidence, the World Health Organization (WHO) and Food and Agriculture Organization Joint Expert Committee on Food Additives (JECFA) confirmed its existing stance that aspartame consumption of up to 40 mg per kg of body weight per day — the amount found in 9 to 14 diet soft drinks — is safe.

The decision, which was anticipated by Reuters in late June, drew praise from an array of experts who weighed in on the study results via the UK-based Science Media Centre. Many emphasized the lack of data showing a causal relationship between the low-calorie artificial sweetener and sought to temper any alarmism related to the decision.

“In short the evidence that aspartame causes primary liver cancer, or any other cancer in humans, is very weak,” said Paul Pharoah, MD, PhD, a professor of cancer epidemiology at Cedars-Sinai Medical Center, Los Angeles, California. “Group 2B is a very conservative classification in that almost any evidence of carcinogenicity, however flawed, will put a chemical in that category or above.”

Other examples of substances classified as Group 2B are extracts of aloe vera, diesel oil, and caffeic acid found in coffee and tea, Pharoah explained, adding that “[t]his is reflected in the view of the [JECFA] who concluded that there was no convincing evidence from experimental animal or human data that aspartame has adverse effects after ingestion.”

“The general public should not be worried about the risk of cancer associated with a chemical classed as Group 2B by IARC,” he stressed.

Alan Boobis, OBE, Ph.D., similarly noted that the Group 2B classification “reflects a lack of confidence that the data from experimental animals or humans is sufficiently convincing to reach a clear conclusion that aspartame is carcinogenic.”

“Hence, exposure at current levels would not be anticipated to have any detrimental effects,” added Boobis, emeritus professor of toxicology, at Imperial College London in England.

The IARC/JECFA opinion is “very welcome” and “ends the speculation about the safety of aspartame,” added Gunter Kuhnle, a professor of nutrition and food science at the University of Reading in England.

“Unfortunately, leaking some information might have created unnecessary uncertainty and concern as consumers might be rightfully worried if they are told that something that is in many foods could cause cancer,” Kuhnle said. “The published opinion puts this into perspective and makes it very clear that there is no cause for concern when consumed at the current amounts.”

The data reviewed by the IARC Working Group included three studies, comprising four prospective cohorts, that “assessed the association of artificially sweetened beverage consumption with liver cancer risk,” the group reported in The Lancet.

The cohort studies — including one conducted within 10 European countries, one that pooled data from two large US cohorts, and a prospective study also conducted in the US — each “showed positive associations between artificially sweetened beverage consumption and cancer incidence or cancer mortality” in the overall study population or relevant subgroups.

Although the studies were of “high quality and controlled for many potential confounders,” the Working Group concluded that “chance, bias, or confounding could not be ruled out with reasonable confidence.” Thus, the evidence for cancer in humans was deemed “limited” for hepatocellular carcinoma and “inadequate” for other cancer types,” the group explained.


In June 2023, a Working Group of 25 scientists from 12 countries met at the International Agency for Research on Cancer (IARC) in Lyon, France, to finalize their evaluation of the carcinogenicity of aspartame, methyl eugenol, and isoeugenol. Aspartame was classified as “possibly carcinogenic to humans” (Group 2B) based on “limited” evidence for cancer in humans. There was also “limited” evidence for cancer in experimental animals and “limited” mechanistic evidence. Methyl eugenol was classified as “probably carcinogenic to humans” (Group 2A) based on “sufficient” evidence for cancer in experimental animals and “strong” mechanistic evidence, including studies in humanized mice and supported by mechanistic studies in exposed humans. Isoeugenol was classified as “possibly carcinogenic to humans” (Group 2B) based on “sufficient” evidence for cancer in experimental animals. For both methyl eugenol and isoeugenol, the evidence regarding cancer in humans was “inadequate”, as no epidemiological studies were available. These assessments will be published in Volume 134 of the IARC Monographs.

Immediately following IARC’s meeting on cancer hazard identification, the Joint FAO/WHO Expert Committee on Food Additives (JECFA) conducted a risk assessment exercise, including a review of the acceptable daily intake of aspartame. A summary of these results has been published.

Aspartame is a low-calorie artificial sweetener widely used in foods and beverages since the 1980s. The highest concentrations are found in tabletop sweeteners, chewing gums, and food supplements; historically, artificially sweetened beverages have been the major source of aspartame exposure (>90% of total exposure in some populations). Currently, artificially sweetened beverages remain an important source of aspartame exposure, but aspartame is typically used in mixtures with other sweeteners. Other sources of aspartame exposure include cosmetics and medicines. Occupational exposure by inhalation during the production of aspartame-containing products has been reported, but data are sparse. Available information indicates that the metabolism of aspartame is similar in humans and experimental systems; aspartame is hydrolyzed to aspartic acid, to the essential amino acid phenylalanine, and methanol. In experimental systems (primates), aspartic acid and methanol are predominantly excreted as CO2; however, most of the phenylalanine is retained.
For cancer in humans, there was “limited” evidence that aspartame causes hepatocellular carcinoma. Prospective cohort studies assessing consumption of artificially sweetened beverages in periods and countries in which artificially sweetened beverages predominantly contained aspartame and were the main source of aspartame exposure were considered informative for the evaluation because artificially sweetened beverage consumption was judged to be a reliable proxy for aspartame exposure. The NutriNet-Santé study is the only large prospective cohort study that comprehensively assessed aspartame exposure from all dietary sources.

Although this study reported an association of aspartame with increased breast, obesity-related, and overall cancer risk, such findings were not consistent across all available studies. The NutriNet-Santé study did not investigate the association of aspartame with liver cancer risk. The Working Group identified three studies, comprising four prospective cohorts, that assessed the association of artificially sweetened beverage consumption with liver cancer risk. These included a large cohort study, conducted within ten European countries, that assessed the association of artificially sweetened beverages with the incidence of hepatocellular carcinoma;  a second study, pooling data from two large US cohorts, that investigated the association between artificially sweetened beverage consumption and liver cancer incidence by diabetes status;  and another large US prospective cohort study that evaluated the association between artificially sweetened beverages and liver cancer mortality.

 Among all three studies, positive associations between artificially sweetened beverage consumption and cancer incidence or cancer mortality were reported in the overall study population or i relevant subgroups.

All three studies were of high quality and controlled for many potential confounders. However, the Working Group concluded that chance, bias, or confounding could not be ruled out with reasonable confidence in this set of studies. Thus, the evidence for cancer in humans was deemed “limited” for hepatocellular carcinoma and “inadequate” for other cancer types.

 The Working Group evaluated several carcinogenicity studies in multiple species (mouse, rat, dog, and hamster), including regulatory study reports made publicly available by the European Food Safety Authority, which reported negative findings after oral exposure to aspartame. It was noted that several of the negative studies were conducted before the advent of Good Laboratory Practice (GLP) guidelines and had some limitations—eg, lack of information on the test substance purity and selective histopathology. No significant increase in the incidence of tumors was observed in three well-conducted GLP studies in male and female transgenic mice.

The Working Group noted that these new transgenic mouse models may not have been sufficiently sensitive to detect a carcinogenic effect of chronic aspartame exposure. In Swiss mice and Sprague-Dawley rats exposed perinatally followed by postnatal oral administration (feed), aspartame caused hepatocellular carcinoma, hepatocellular adenoma, or hepatocellular carcinoma (combined), bronchioloalveolar carcinoma, bronchioloalveolar adenoma or carcinoma (combined), lymphoblastic leukemia, monocytic leukemia, and total myeloid tumors in male mice; lymphoblastic leukemia and leukemia (all types) in female mice; malignant schwannoma in male rats; and mammary gland carcinoma, renal pelvis papilloma, and leukemia (all types) in female rats.

 In Sprague-Dawley rats exposed by oral administration (feed), aspartame caused renal pelvis and ureter carcinoma, renal pelvis and ureter papilloma or carcinoma (combined), and mammary gland carcinoma in females; and monocytic leukemia, histiocytic sarcoma, and total myeloid tumors in males.

Because of concerns regarding some of the diagnoses for lymphomas and related combinations and other lymphoid proliferations in these studies, the Working Group focused its evaluation on all the other neoplastic lesions. Although the data from the above studies  suggested that aspartame had carcinogenic activity, overall, the Working Group considered the evidence for cancer in experimental animals to be “limited” because of questions about the adequacythe  of the design, conduct, interpretation, and reporting of each of the studies. For example, the lack of adjustment for litter effects  may have led to false positive results for incidence and trend. A minority of the Working Group did not share these concerns about this set of studies and considered the evidence for cancer in experimental animals to be “sufficient”; thus, they supported a Group 2A classification rather than Group 2B classification for aspartame.

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Lancet. Published online July 13, 2023. Abstract 


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