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Is Aspartame a Possible Carcinogen?

WHO Aspartame cancer

Aspartame, the primary artificial sweetener found in many diet sodas like Coke and some Snapple drinks, is making headlines once again. However, this time it’s a bit more serious, because the International Agency for Research on Cancer (IARC), which is an extension of the World Health Organization (WHO), is going to be listing aspartame as a “possible carcinogen to humans” later this month, which could be a huge deal.

In this article, we are going to:

  • Discuss the claims that the IARC is going to make, and what they mean
  • Discuss the basis for these claims
  • Review the shortcomings of these claims and why they’re not rooted in good scientific evidence
  • Provide practical recommendations to help you decide whether or not you should avoid aspartame

What is IARC and what are they claiming?

The IARC is a specialized entity of the WHO that evaluates the potential causes of cancer and determines the carcinogenicity of substances and agents. Groups of experts come together to classify substances as carcinogenic to humans, probably carcinogenic, possibly carcinogenic, or not classifiable based on the currently available scientific literature.

The four categories are defined as follows:

  • Carcinogenic to Humans: This classification is assigned when there is sufficient evidence from human studies to conclude that exposure to the substance or agent can cause cancer. The evidence shows a clear link between the substance and an increased risk of developing cancer in humans.
  • Probably Carcinogenic: This classification is used when there is strong evidence suggesting that the substance or agent has the potential to cause cancer in humans, but the available data may be limited or not entirely conclusive. While the evidence is not as definitive as for the "carcinogenic to humans" category, it still suggests a significant likelihood of a causal association between the substance and cancer.
  • Possibly Carcinogenic: This classification is applied when there is some evidence indicating that the substance or agent could potentially cause cancer in humans. However, the evidence is typically weaker or more uncertain compared to the "probably carcinogenic" category. More research is needed to draw definitive conclusions regarding its carcinogenicity.
  • Not Classifiable: This classification is assigned when the available evidence is either insufficient or of inadequate quality to determine the substance's carcinogenicity to humans. It does not imply that the substance is proven to be safe, but rather that there is a lack of conclusive evidence to support a definitive classification as either carcinogenic or non-carcinogenic. Further research may be required to make a more accurate determination.

The purpose of the IARC is to provide important references for policymakers and the scientific community in understanding cancer risks in an attempt to improve global cancer prevention.

Later this month, the IARC is supposed to classify Aspartame as “possibly carcinogenic to humans,” which could potentially impact how various food products are produced.

What does this actually mean?

One of the biggest issues with these kinds of headlines is that people read them and immediately run to social media and yell out “Aspartame causes cancer, I told you so!” without even knowing what they mean.

First, it’s important to note that the IARC claims are based on assessing the evidence for the ability of a substance to cause cancer. This means that IARC's classifications are based on the strength of the scientific evidence linking a substance to cancer, without considering factors like the level of exposure or the actual risk posed to individuals.

For this reason, the IARC’s claims do not directly indicate the likelihood or magnitude of risk associated with exposure to that substance. Different variables like the concentration or duration of exposure, the route of exposure, and individual susceptibility to the substance can all influence the actual risk of the substance, which is why the IARC’s claims on their own don’t mean a whole lot.

It is critically important to differentiate between the classification of a substance by IARC in terms of its potential to cause cancer and the actual risk it poses in real-world scenarios. The evaluations provided by IARC simply serve as a foundation for further risk assessments, which consider exposure levels and other contextual factors to provide a more comprehensive understanding of the potential risks associated with a particular substance.

Next, the IARC is not claiming that aspartame causes cancer. Instead, they are claiming that aspartame may be a possible carcinogen.

The “possible carcinogen” category is one of the weakest categories of classifications for carcinogenicity.

In essence, a possible carcinogen as defined by the IARC is a substance where there is limited evidence of carcinogenicity in humans and less than sufficient evidence of carcinogenicity in experimental animals. Oftentimes, claims for “possible carcinogens” are based on studies that have conflicting or inconclusive results. In other words, there isn’t strong evidence in humans to suggest that the substance is carcinogenic at all.

At best, it’s an “educated guess.” However, in the case of aspartame, it’s less than an educated guess because we actually have plenty of data suggesting that it’s safe below certain quantities (more on this later) which, again, indicates the lack of nuance with these classifications because they do not take into account dosing.

A big issue with the "possible carcinogen" classification is that it can also lead to the perception that all substances within this category carry equal levels of risk. In reality, the substances grouped under this classification can vary widely in terms of their potency, the extent of exposure required for harm, or the presence of confounding factors.

Here are some examples of things listed by the IARC that are more carcinogenic than Aspartame, which perfectly depicts the shortcomings of these classifications - red meat, drinking beverages hotter than 65 degrees Celsius, and occupational exposure as a hairdresser. All of these things are considered more carcinogenic than aspartame.

See the issue?

The classifications are SO VAGUE.

Saying that something is linked to disease really doesn’t tell us anything if we don’t quantify a harmful dose.

It’s the same as saying that drinking water can kill you. This is technically a true statement, but it lacks nuance.

How much water would you have to drink to die?

The issue is that the statement itself “aspartame is possibly carcinogenic” causes a ton of fear because people do not understand these nuances and even worse, media outlets purposefully leave them out, so their articles get more clicks!

What does the scientific evidence actually say?

Now that we’ve discussed why the IARC’s classifications are faulty to being with, we should discuss what the research on aspartame and cancer actually shows.

Most of the research studies showing that aspartame can cause cancer were conducted in animals, not humans. Now, this doesn’t immediately discredit these studies, but we cannot make conclusive claims about the effects of a substance in humans if the only available causal evidence is demonstrated in animals.


For starters, our biology is different. There are substances that are toxic to humans that are not toxic to certain animals, and vice versa. Second, and most importantly, the doses that are given to these animals are insanely high and not realistic whatsoever when translated to normal aspartame consumption in humans.

For example, the FDA establishes the “no-adverse-effect-levels” (NOAEL) for different substances, which is the highest tested dose is of a substance that does not produce any detectable adverse effects. These levels are determined based off toxicological studies in animal models where they provide animals different doses of a particular substance to determine how much is too much essentially.

The NOAEL of aspartame is set to 4000 mg/kg bw/day.

To translate this to numbers that you’ll understand – a 175lb adult would have to drink about 1600 diet cokes in one day to reach that amount.

That’s not a typo. Yes, it’s really 1600 diet cokes.

See why these animal studies are not really translatable to humans?

Now, there are some observational studies in humans showing an association between aspartame consumption and certain types of cancer, but the data is mixed and not conclusive. Further, you can't determine causality in an observational study.

For example, an observational study conducted in over 100,000 adults in France was published last year and found that those who consumed larger amounts of artificial sweeteners, aspartame included, had a slightly higher risk of developing cancer [1]. However, there are a number of different issues with this study.

First, there’s the risk of selection bias. Selection bias is a limitation of this study that refers to the possibility that the participants who were included in the analysis may not be representative of the general population. For example, the study relied on self-reported dietary data, which may be subject to recall bias or misreporting. This could lead to an over or underestimation of the true associations between artificial sweeteners and cancer risk.

Next, there’s the potential effect of residual confounding variables, which refers to the possibility that the observed associations between artificial sweeteners and cancer risk may be influenced by other factors that were not measured or accounted for in the analysis. For example, the study did not account for potential differences in lifestyle or health behaviors between artificial sweetener consumers and non-consumers, which could confound the observed associations.

Strengthening this point is an observational study that showed no association between aspartame and cancer in the general population, except in individuals who were diabetic [2]. We know that diabetes increases the risk of cancer, and perhaps, individuals with diabetes have more diet beverages because they want to reduce their sugar intake. This is speculative, of course, but it’s a perfect example of a potential confounding variable.

Lastly, there’s the possibility of reverse causality. This is a limitation in all associative studies. Correlation does not equal causation. We automatically assume that it’s aspartame that causes cancer when we see these associations in studies, but it could very well be that individuals who are at a higher risk for cancer, AKA make lifestyle choices that increase the risk of cancer (e.g., smoking), are more likely to consume diet drinks as well.

On top of all this, there are data showing that aspartame consumption is not associated with cancer. The 2023 study that we just cited didn’t find an association between aspartame and cancer, only in diabetic adults and we already discussed what some potential confounding variables are there. Aside from this study, there’s a large-scale meta-analysis that was published last year on aspartame and cancer that analyzed data from over 25 observational studies which included a grand total of 3,739,775 participants. The data showed that artificial sweetener consumption has no association with cancer or all-cause mortality [3].

Overall, there just isn’t any strong evidence to conclude that drinking diet beverages containing aspartame will cause cancer, unless perhaps you’re able to chug over 1500 cans of diet soda per day.

What does this mean for you?

Now that we've examined the evidence and the shortcomings of the IARC's classification, let's discuss what this means for you, the consumer.

First and foremost, we want to make it clear that we are not advocating for the consumption of diet soda or any other artificially sweetened beverages. We just want to provide accurate information so that you can make an informed decision. It's simply not accurate to claim that aspartame is inherently harmful based on the IARC's classification.

Furthermore, artificial sweeteners may actually have benefits for weight loss. There is plenty of data showing that artificial sweeteners, including aspartame, can help individuals lose weight and maintain it when compared to water [4][5][6]. The reason why, might be that these sweeteners aid in curbing cravings for sugary foods, thus leading to a reduction in overall calorie intake. We know that obesity increases the risk of not only cancer but many other diseases including diabetes and CVD. So, in some cases, artificial sweeteners may actually provide an overall net benefit to someone’s health.

Ultimately, the decision of whether or not to consume aspartame-containing products is a personal one. It's essential to consider your own health goals, preferences, and overall dietary patterns. If you enjoy diet beverages and consume them in moderation, the current scientific evidence does not suggest that you need to be overly concerned about their safety.

Overall, while the IARC's classification of aspartame as a "possible carcinogen" may have sparked fear and concern, it's crucial to understand the nuances of these classifications and the limitations of the evidence. The available data, including large-scale studies and meta-analyses, do not provide strong evidence to support the notion that consuming reasonable amounts of aspartame poses a significant risk of cancer. As with any dietary choice, it's always wise to make informed decisions based on a balanced understanding of the scientific evidence and your own individual circumstances.


1. Debras, C., et al., Artificial sweeteners and cancer risk: Results from the NutriNet-Santé population-based cohort study. PLoS Med, 2022. 19(3): p. e1003950.

2. Palomar-Cros, A., et al., Consumption of aspartame and other artificial sweeteners and risk of cancer in the Spanish multicase-control study (MCC-Spain). Int J Cancer, 2023.

3. Yan, S., et al., Can Artificial Sweeteners Increase the Risk of Cancer Incidence and Mortality: Evidence from Prospective Studies. Nutrients, 2022. 14(18).

4. Laviada-Molina, H., et al., Effects of nonnutritive sweeteners on body weight and BMI in diverse clinical contexts: Systematic review and meta-analysis. Obes Rev, 2020. 21(7): p. e13020.

5. Peters, J.C., et al., The effects of water and non-nutritive sweetened beverages on weight loss and weight maintenance: A randomized clinical trial. Obesity (Silver Spring), 2016. 24(2): p. 297-304.

6. Peters, J.C., et al., The effects of water and non-nutritive sweetened beverages on weight loss during a 12-week weight loss treatment program. Obesity (Silver Spring), 2014. 22(6): p. 1415-21.

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