The National Institute for Health and Clinical Excellence has released new guidelines on treating children with gastroenteritis, a stomach bug that causes sickness and diarrhoea.
The guidelines say that children should not drink fruit juices or fizzy drinks when they are suffering from this virus - even though this is a remedy many people have sworn by for years.
Instead, if they become dehydrated they should take special oral rehydration solutions, such as Dioralyte, and drink plenty of water.
Between 10% and 20% of children under 5 years old go to the doctor with symptoms of gastroenteritis each year, and many more are treated at home. They usually recover quickly.
Dr Stephen Murphy, who specialises in child stomach problems and worked on the new guidelines said: 'Most children can be looked after at home but if a child becomes dehydrated they may need to go to hospital.
'The idea that flat cola and lemonade helps is just a myth. In fact, it can make it worse, but unfortunately people are still using them.'
The guidelines also say it's important to keep washing your hands with soap after going to the toilet, changing nappies and preparing food as this will help to stop the virus spreading.
Sources: BBC: Sugary drinks 'worsen vomit bug'
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More information and advice on gastroenteritis