Experts say that kids suffering from diarrhea and vomiting shouldn't be given fizzy drinks, for they could make their condition worse.
The National Institute for Health and Clinical Excellence (NICE) said the idea that flat drinks can help with gastroenteritis is untrue, while fruit juices and fizzy drinks do not help either.
NICE suggested that bad cases of stomach bugs in children under five should in fact be treated with rehydration drinks.
NICE made the warning as part of guidance it has produced on the treatment of gastroenteritis in children in England and Wales.
"The idea that flat coke and lemonade, or fruit juices for that matter, helps is just a myth. In fact, it can make it worse, but unfortunately people are still using them," the BBC quoted consultant pediatric gastronenterologist Dr Stephen Murphy, who chaired the panel drawing up the guidance, as saying.
"Severe cases of diarrhea and vomiting leading to dehydration need treating with oral rehydration solution immediately," he added.
He suggested that the combination of sugar and salt in rehydration drinks was the key to helping the body absorb fluids, whereas the likes of coke and lemonade had too much sugar.