The WHO said Friday it was now classifying aspartame, an artificial sweetener commonly used in soft drinks, as "possibly carcinogenic to humans" - though the acceptable daily intake level remains unchanged.
"We're not advising companies to withdraw products, nor are we advising consumers to stop consuming altogether," said Francesco Branca, the World Health Organization's nutrition and food safety director.
"We're just advising for a bit of moderation," he told a press conference presenting the findings of two reviews of available evidence on aspartame.
The WHO's International Agency for Research on Cancer (IARC) carried out its first-ever evaluation of the carcinogenicity of aspartame at a meeting in Lyon, France, from June 6 to 13.
"The working group classified aspartame as possibly carcinogenic to humans," the WHO said.
It was placed in category Group 2B, based on the limited evidence available, which specifically concerned hepatocellular carcinoma - a type of liver cancer.
There was also limited-strength evidence regarding cancer in experimental animals.
The Group 2B category also contains extract of aloe vera and caffeic acid found in tea and coffee, said Paul Pharoah, a professor of cancer epidemiology at the Cedars-Sinai Medical Center in Los Angeles.
"The general public should not be worried about the risk of cancer associated with a chemical classed as Group 2B," he said.
The IARC's Mary Schubauer-Berigan said the limited evidence for hepatocellular carcinoma came from three studies, conducted in the United States and across 10 European countries.
"These are the only epidemiological studies that examined liver cancer," she told reporters.
Branca added: "We have, in a sense, raised a flag here, indicating that we need to clarify much more the situation," but nor is it "something which we can dismiss".
9-14 cans a day
A second group, JECFA - the Joint Expert Committee on Food Additives formed by the WHO and its fellow UN agency the Food and Agriculture Organization - met in Geneva from June 27 to July 6 to evaluate the risks associated with aspartame.
It concluded that the data it evaluated indicated no reason to change the acceptable daily intake (ADI), established in 1981, of zero to 40 milligrammes of aspartame per kilogramme of body weight.
With a can of sugar-free soft drink typically containing 200 or 300 mg of aspartame sweetener, an adult weighing 70 kg would therefore need to consume more than nine to 14 cans per day to exceed the ADI, assuming no additional aspartame intake from other sources.
"The problem is for high consumers," said Branca.
"Somebody who drinks a soda every once in a while... shouldn't have a concern."
Sodas, gum and cereals
Aspartame is an artificial chemical sweetener widely used in various food and beverage products from the 1980s onwards.
It is found in diet drinks, chewing gum, gelatin, ice cream, dairy products such as yoghurt, breakfast cereals, toothpaste, cough drops and chewable vitamins.
The International Sweeteners Association said that Group 2B classification puts aspartame in the same category as kimchi and other pickled vegetables.
"JECFA has once again reaffirmed aspartame's safety after conducting a thorough, comprehensive and scientifically rigorous review," said ISA chief Frances Hunt-Wood.
But for Camille Dorioz, campaign manager at the consumer organisation Foodwatch, Friday's update leaves a "bitter taste".
"A possibly carcinogenic sweetener has no place in our food and drink," he said.
'Drink water'
Back in May, the WHO said artificial sweeteners, used to replace sugar in a vast range of products, do not help in losing weight and can have serious health effects.
The UN health agency released guidelines advising against using so-called non-sugar sweeteners.
Branca was asked about what consumers should do in the light of Friday's update, when trying to choose what was best between a soft drink with added sugar, and one with added sweeteners.
"There should be a third option considered, which is to drink water instead - and to limit the consumption of sweetened products altogether," he replied.
"There are alternatives that do not contain either free sugars or sweeteners - and those should be the products that should be preferred by consumers."
