Aspartame, a popular artificial sweetener found in Diet Coke, chewing gum, yoghurt and other food products, and consumed by millions every day, has long been the topic of fierce debate over its impact on health.
On Friday, the World Health Organization's International Agency for Research on Cancer classified aspartame as "possibly carcinogenic to humans," citing "limited evidence" suggesting a link to cancer.
But the WHO emphasised that it will not change its maximum level of acceptable daily intake of aspartame and that more research is needed.
Here is a brief explainer about the sweetener.
What is aspartame?
Aspartame is a combination of two amino acids, phenylalanine and aspartic.
It was discovered by chance in 1965 by a chemist working for the Searle pharmaceutical company who was trying to find a treatment for ulcers.
Aspartame has a similar level of energy to sugar – one gram has around four calories – but it is around 200 times sweeter.
This means that a far lower amount can be used to replace sugar and sweeten food, while keeping the calorie level down.
Aspartame is not the first nor only artificial sweetener.
Saccharin, first discovered in 1879, is also used to artificially sweeten products, but it has a bitter aftertaste.
Other more natural sweeteners such as stevia, which is derived from plant leaves, have risen in popularity in recent years.
What products use it?
Thousands of products across the world use aspartame – many marketed as diet, light, or zero-calorie.
Aspartame is added to soft drinks, chewing gum, sweets, jelly, yoghurts, dairy products, cough drops, toothpaste and store-bought desserts. It is also used for tabletop sweeteners commonly stirred into tea and coffee.
It is even added to more than 600 drugs as a little sweetness to help the medicine go down.
How did it get approved?
The U.S. Food and Drug Administration first approved the use of aspartame in 1974, in a decision that proved controversial almost immediately.
The next year aspartame's approval was suspended after critics said animal studies suggested it was potentially toxic and carcinogenic.
In 1981, the FDA re-approved its use, saying the amount humans consume was far below the level even suspected of being toxic.
Aspartame was then approved for use in drinks in 1983.
More than 90 countries have since authorised the sweetener.
The WHO and other international health bodies have set the maximum acceptable daily intake for aspartame at 40 milligrams per kilogram of body weight a day.
If one can of diet soft drink contains 200 to 300 milligrams of aspartame, an adult weighing 70 kilograms would have to drink nine to 14 cans a day to exceed their acceptable intake.
Why is it controversial?
Rarely in the history of food has a product fuelled so much debate and controversy.
A study by Italian researchers in 2010 said that aspartame causes liver and lung cancer in male mice.
A Danish study in 2010 meanwhile found an association between artificially sweetened drinks and pregnant women giving birth prematurely.
Some of the previous research conducted on animals has been criticised for giving them far more aspartame than humans would normally consume.
Among humans, most of the studies have been observational, which means they cannot directly point to aspartame as a cause – and other lifestyle factors cannot be ruled out.
After a lengthy review, the European Food Safety Authority in 2013 ruled that aspartame and its related products are safe for the general population, as long as the acceptable daily intake is not exceeded.
In May, the WHO released new guidelines advising against the use of all non-sugar sweeteners, saying that the available evidence suggests they do not help with long-term weight loss and could be linked to an increased risk of diabetes and heart problems.