Artificial sweeteners, used to replace sugar in a vast range of products, do not help in losing weight and can have serious health effects, according to the World Health Organization.
The United Nations' health agency released new guidelines advising against using so-called non-sugar sweeteners, or NSS.
A systematic review of available evidence "suggests that use of NSS does not confer any long-term benefit in reducing body fat in adults or children," WHO said in a statement.
In addition, results from the review "suggest that there may be potential undesirable effects from long-term use of NSS, such as an increased risk of type 2 diabetes, cardiovascular diseases, and mortality in adults".
Sweeteners are consumed by millions every day in products like diet soda or to sweeten coffee, partly as a way to avoid weight gain from sugar -- but how healthy these substitutes are has long been a matter of controversy.
The WHO's director for nutrition and food safety Francesco Branca stressed that replacing regular sugar with artificial sweeteners "does not help with weight control in the long term".
"People need to consider other ways to reduce free sugars intake, such as consuming food with naturally occurring sugars, like fruit, or unsweetened food and beverages," he said in a statement.
"NSS are not essential dietary factors and have no nutritional value. People should reduce the sweetness of the diet altogether, starting early in life, to improve their health."
The WHO said its new recommendation applied to everyone except individuals with pre-existing diabetes.
And it includes all synthetic and naturally occurring or modified sweeteners not classified as sugars found in manufactured foods and beverages or sold on their own to be added to products by consumers.
Among the most widely used sweeteners are acesulfame K, aspartame, advantame, cyclamates, neotame, saccharin, sucralose, stevia and stevia derivatives, it said.
Meanwhile, the UN health agency emphasised that its guidelines were considered conditional since it remained difficult to draw firm conclusions due to the diversity of participants and complexity of NSS-use seen in the studies it had examined.