The clinical evidence is mixed: Some observational studies show that higher levels of dietary aspartame intake are associated with an increased risk of some cancers,[1] while other observational studies do not support that relationship.[2] The current evidence also doesn’t show a dose response between aspartame intake and cancer risk,[1][2] and no long-term randomized controlled trials of aspartame intake have investigated cancer outcomes. Therefore, the direct causality between aspartame exposure and cancer cannot yet be determined.
For these reasons, aspartame has been classified by the International Agency for Research on Cancer (IARC) and the Joint Food and Agriculture Organization and World Health Organisation Expert Committee on Food Additives (JECFA) as a “Group 2B” (possibly carcinogenic) compound in a four-group classification system.[3]
To help put this classification in context, Group 1 means “carcinogenic to humans”, and includes things like solar radiation, processed meat, tobacco smoking, and air pollution. Group 2A means “probably carcinogenic to humans”, and includes eating red meat, anabolic steroids, indoor fireplace emissions, etc. Group 3 means “not classifiable regarding its carcinogenicity to humans”. Interestingly, the classification for aspartame — Group 2B — contains other common foods and supplements including pickled vegetables, aloe vera, kava extract, and caffeic acid (which is found in all plants, including coffee), and many other substances humans are exposed to daily.
Consequently, while aspartame is “possibly carcinogenic” (has the potential to cause cancer), aspartame intake in the amounts that are typically consumed is unlikely to cause cancer in humans (a person weighing 70 kg would have to consume approximately 9–14 cans of diet soda per day to exceed the acceptable daily intake level of 40 mg/kg/day). This is the current message conveyed by food regulatory bodies in the US — the FDA — and in Europe — the EFSA.[4] However, this message might change when more high-quality clinical studies are completed.
It’s also important to note that it is challenging to prove that foods cause cancer in humans, because much of the evidence relies on observational studies that are unable to prove causality. For example, observational studies have shown that most common food ingredients are associated with both an increased and a decreased cancer risk, whereas meta-analyses of such studies find that the claimed benefits of such foods are nonexistent.[5]
To summarize, the current evidence does not show that aspartame causes cancer in humans but the evidence doesn’t rule out that it can — absence of evidence is not evidence of absence. While aspartame is unlikely to pose a major risk, it is a food additive and not an essential nutrient; therefore, people can choose to avoid foods that contain aspartame to remove the exposure to any potential hazard.