Coca-Cola and Pepsi are changing the recipes for their soft drinks to avoid being forced by law to put a cancer warning on the label.
The caramel colouring in the drinks will contain lower levels of 4-methylimidazole, which has been added to the list of carcinogens in California law.
Coca-Cola's recipe is being changed across the US - but will not be changed in Britain or the rest of Europe.
Under the legislation, drinks containing a certain level of carcinogens must bear a cancer warning label.
Coca-Cola and PepsiCo account for almost 90% of the fizzy drinks market in America.
The changes have already been made for drinks sold in California.
The companies said the new recipes would be rolled out across the US to streamline their manufacturing processes.
Diana Garza-Ciarlante, a spokeswoman for Coca-Cola, said: "While we believe that there is no public health risk that justifies any such change, we did ask our caramel suppliers to take this step so that our products would not be subject to the requirement of a scientifically unfounded warning."
The company later said that while its manufacturers had been asked to change the caramel ingredient, its "secret formula" was not being altered.
The American Beverage Association said the new standard was based on one study linking the chemical to cancer in mice and rats, but added there was no evidence showing that it causes cancer in humans.
The Food and Drug Administration said a person would have to drink more than 1,000 cans a day to reach the doses administered that have shown links to cancer in rodents.