Last week a consumer advocacy group reported that the leading brown sodas contain levels of 4-methylimidazole (4-MI) – an animal carcinogen – high enough to cause cancer in 7 out of 1 million Americans. Days later, soda companies, including Coca-Cola and Pepsi, announced that they were reducing the amount of 4-MI in their colas to meet the limit set by California – the only state that regulates the substance.


However industry says it did not make this change for public health reasons, since the amount of 4-MI in colas does not actually pose a risk to consumers. Instead, companies want to avoid having to put a cancer warning label on cans in the event that government regulators decide to restrict 4-MI. 

But this change in policy, requested by the Center for Science in the Public Interest (CSPI) seems unlikely to come about anytime soon, as even the Food and Drug Administration (FDA) refuted the group’s claims that the 138 micrograms it found in brown sodas on average poses a cancer risk. 

“A person would have to drink more than a thousand cans of soda in a day to match the doses administered in studies that showed links to cancer in rodents,” said FDA spokesperson Doug Karas in a now widely-cited statement. 

According to Dr. James Coughlin, a toxicologist who studies animal carcinogen, the risk posed by 4-MI is even smaller than this government estimate suggests. 

The limit on 4-MI now being adopted by cola companies nationwide was set by California’s Proposition 65, which lists all chemicals considered carcinogens by the state. 

To set this threshold, rulemakers relied on the results of a 2007 study from the National Toxicology Program which found that 4-MI at high doses caused lung cancer in mice.


But in order for humans to reach the equivalent of even the lowest cancer-causing dose in mice, says Coughlin, a woman would have to drink 37,000 cans (12 oz) a day for the rest of her life. A man, on the other hand, would have to drink a whopping 95,000 cans a day during his lifetime. These figures are taken from a slightly more conservative study of colas done last year that found an average of 130 micrograms of 4-MI per can as opposed to the 138 micrograms found by CSPI. 

“It’s certainly not a health risk,” Coughlin told Food Safety News. “Cola is not causing cancer in humans. It’s just not happening.” 

In fact, he says, in the same study, rats were also given high doses of 4-MI, and none of them developed tumors. The chemical even reduced their risk of 5 other types of cancer (besides the lung cancer it produced in mice).

“I believe this is much ado about nothing,” he says. 

But Michael Jacobson, executive director of CSPI, says the organization is sticking by its petition to limit 4-MI nationwide. FDA’s claim that one would need to drink over 1,000 cans a day to risk cancer is “a ridiculous statement,” he says, “and it deserves ridicule.” 

“It’s disappointing to see FDA defending a cancer-causing chemical in the food supply,” he said in an interview with Food Safety News.

Jacobson says it’s not the dose but the percentage of mice that got tumors in the study that matters.  Since about 30 percent of the mice developed cancer, and a can of soda has about 1/1,000 of the amount of 4-MI administered to the mice, that would equate to 30 cancer cases in every 100,000 people. 

Coughlin, on the other hand, says you can’t apply the number of illnesses in an animal study directly to humans. 

“CSPI took these animal numbers and calculated cases of human cancer, but you can’t just take those animal statistics and transfer them,” he says. 

For this reason, adding 4-MI to California’s list of toxins was a mistake in the first place, he says, as it was based on the prevalence of cancer in mice and not adjusted properly for the human body.

“There is strong scientific evidence that the chemical never should have been listed by Prop-65.”

Nonetheless, Coke and Pepsi – whose products had the most 4-MI in CSPI’s study – are changing how they produce caramel coloring (this process is what creates 4-MI) in order to avoid backlash should nationwide policy adapt to California law.


“The companies that make caramel coloring for our members’ soft drinks are now producing it to meet California’s new standard, and it will be used in products nationwide,” said the American Beverage Association in a statement. “Consumers will notice no difference in our products and have no reason at all for any health concerns, as supported by FDA and regulatory agencies around the world.”

Editor’s note: An earlier version of this article quoted James Coughlin as saying “I believe this is much ado about something,” when in fact he said “I believe this is much ado about nothing.” The story has been updated to reflect this correction.