Sixty years after a chemist at a pharmaceutical company accidentally licked his finger and discovered something sweet, aspartame remains one of the most debated ingredients in the American food supply. This summer, as millions of Americans reach for a cold diet cola, the ingredient in that can has outlasted two presidential administrations, a contentious FDA approval process, and a 2023 World Health Organization classification that managed to say two opposite things at the same time.
Aspartame was discovered in 1965 by James Schlatter, a G.D. Searle chemist testing an anti-ulcer drug. He noticed an intensely sweet taste on his finger, traced it to a compound he had synthesized, and set in motion what would become a billion-dollar sweetener industry. G.D. Searle petitioned the FDA for approval in 1973. The process took eight years. In 1980, the FDA’s own Public Board of Inquiry voted unanimously to reject aspartame pending further study, citing brain tumor concerns in animal trials. Three of six FDA scientists assigned to review the data advised against approval. In July 1981, FDA Commissioner Arthur Hayes — appointed the day after Ronald Reagan’s inauguration, with G.D. Searle CEO Donald Rumsfeld on the presidential transition team — overruled the board and approved aspartame for dry foods. Carbonated beverages followed in 1983. Hayes resigned later that year and joined Searle’s public relations firm. A 1987 General Accounting Office investigation reviewed the approval process and concluded the FDA had acted properly.
The FDA has since reviewed more than 100 studies on aspartame covering carcinogenicity, reproductive effects, and metabolism. Its position has not changed: aspartame is safe for the general population within the approved acceptable daily intake of 50 milligrams per kilogram of body weight. For a 150-pound adult, reaching that threshold requires approximately 20 cans of diet soda per day. The European Food Safety Authority sets the ADI slightly lower at 40 mg/kg, but its conclusion is the same.
In July 2023, the World Health Organization published two findings on the same day that appeared to contradict each other. The International Agency for Research on Cancer classified aspartame as Group 2B — “possibly carcinogenic to humans” — based on limited evidence, specifically for hepatocellular carcinoma, a type of liver cancer. On the same day, the WHO’s Joint FAO/WHO Expert Committee on Food Additives reaffirmed the existing acceptable daily intake, finding no reason to change it. The FDA rejected the IARC classification outright. The apparent contradiction dissolves when the two agencies’ mandates are understood: IARC identifies whether something can cause harm under some conditions; JECFA assesses whether dietary exposure at real-world consumption levels poses a health risk. Group 2B is the third of four classification levels and includes aloe vera extract, engine exhaust, and more than 300 other substances. A 2025 peer-reviewed mini-review in the Journal of Xenobiotics concluded the carcinogenic evidence remains inconclusive, with studies producing inconsistent results.
Not all diet colas carry the same ingredient profile. Diet Coke uses aspartame alone. Coke Zero Sugar, Diet Pepsi, and Pepsi Zero Sugar use aspartame combined with acesulfame potassium, known as Ace-K. Ace-K is a separate high-intensity sweetener approved by the FDA in 1988 after a review of more than 90 studies. The European Food Safety Authority completed a full re-evaluation of Ace-K in 2025 and reaffirmed its safety, raising the acceptable daily intake to 15 mg/kg. When two sweeteners are combined, manufacturers use less of each to achieve the same sweetness level. Whether that combination changes the risk profile is not established by current research. The IARC Group 2B flag sits on aspartame, not Ace-K. Ace-K carries its own open research questions — a 2024 study flagged a possible link to early puberty onset in adolescent girls, and some research has raised metabolic syndrome associations — but neither finding has been established as a harm, and neither has changed the regulatory position of any major agency.
One population has a clear, established, non-debated reason to avoid aspartame: people with phenylketonuria, a rare inherited disorder that prevents the body from metabolizing phenylalanine, one of aspartame’s breakdown products. Elevated phenylalanine can cause brain damage. Federal law requires every product containing aspartame to carry a label warning for people with PKU. That warning is not a controversy. It is a medical fact.
President Donald Trump reportedly drinks approximately 12 Diet Cokes per day — a habit documented by the New York Times in 2017 and confirmed by the presence of a Diet Coke button installed on the Oval Office desk. At 200 to 300 milligrams of aspartame per can, 12 cans places him below the FDA’s acceptable daily intake for his body weight. Elon Musk has also publicly declared himself a Diet Coke devotee.
The science does not clear aspartame of all questions. It establishes that six decades of study have not confirmed harm at normal consumption levels, that regulators in the United States and Europe continue to approve it, and that the most significant recent classification came with a simultaneous reaffirmation that dietary exposure is not a health concern. What Americans put in their glass this summer is their decision.
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