The sweetener aspartame is a “possible carcinogen” but it remains safe to consume in moderation and at already agreed levels, two groups linked to the World Health Organization (WHO) have declared.

In reviews released early on Friday, the WHO’s cancer agency deemed the sweetener – which is found in diet drinks and countless other foods – as a “possible” cause of cancer, while a separate expert group looking at the same evidence said it still considers the sugar substitute safe in limited quantities.

One review came from the International Agency for Research on Cancer (IARC), a special branch of the WHO. The other report was from an expert panel selected by WHO and another UN group, the Food and Agriculture Organization (FAO).

The guidance on the use of the sweetener remained unchanged.

“We’re not advising consumers to stop consuming [aspartame] altogether,” the WHO’s nutrition director Dr Francesco Branca said on Friday.

“We’re just advising a bit of moderation,” he said.

In a press conference ahead of the announcement, Branca tried to help consumers make sense of the seemingly conflicting declarations, especially those who seek out artificial sweeteners to avoid sugar.

“If consumers are faced with the decision of whether to take cola with sweeteners or one with sugar, I think there should be a third option considered – which is to drink water instead,” he said.

In its first declaration on the additive, the Lyon-based IARC said aspartame was a “possible carcinogen”. That classification means there is limited evidence that a substance can cause cancer.

It does not take into account how much a person would need to consume to be at risk, which is considered by a separate panel, the WHO and FAO Joint Expert Committee on Food Additives (JECFA), based in Geneva.

After undertaking its own comprehensive review, JECFA said on Friday that it did not have convincing evidence of harm caused by aspartame, and continued to recommend that people keep their consumption levels of aspartame below 40mg/kg a day.

It first set this level in 1981, and regulators worldwide have similar guidance for their populations.

Several scientists not associated with the reviews said the evidence linking aspartame to cancer is weak. Food and beverage industry associations said the decisions showed aspartame was safe and a good option for people wanting to reduce sugar in their diets.

The WHO said that the existing consumption levels meant, for example, a person weighing between 60-70kg (132-154 lbs) would have to drink more than 9-14 cans of soft drinks daily to breach the limit, based on the average aspartame content in the beverages.

“Our results do not indicate that occasional consumption could pose a risk to most consumers,” Branca said.

He said the WHO is not urging companies to remove aspartame from their products entirely but is instead calling for moderation from both manufacturers and consumers.

In a statement announcing the assessment results, Branca noted that cancer is a leading cause of death globally with one in six people succumbing to the illness each year.

“Science is continuously expanding to assess the possible initiating or facilitating factors of cancer, in the hope of reducing these numbers and the human toll,” he said.

“The assessments of aspartame have indicated that, while safety is not a major concern at the doses which are commonly used, potential effects have been described that need to be investigated by more and better studies,” he added.