The NHS has issued new guidelines that advise parents not to give fizzy drinks, flat cola drinks or fruit juices to children with diarrhoea and vomiting.
Experts from the National Institute for Health and Clinical Excellence (NICE) said the idea that such drinks help with gastroenteritis is untrue.
Instead, parents should ensure their children are kept hydrated with the right kinds of fluids, such as water with an oral rehydration salt solution.
The new guidance is aimed at managing gastroenteritis in young children, as figures show between 10% and 20% (150,000 to 300,000) of children under five in England are taken to their GP or hospital each year with the symptoms.
Most children recover quickly without treatment and can be looked after at home, and the guidelines recommend that children who are not yet dehydrated should continue to be breastfed if they usually are.
Some youngsters can end up dehydrated and need hospital care. The report says fluids should be kept up, but the drinking of fruit juices and carbonated drinks should be "discouraged".
Children who are dehydrated need a higher level of care, but fruit juices and fizzy drinks should still not be given, it adds.