The effectiveness of some over-the-counter cough and cold medicines for children under 12 has been placed in doubt by a government watchdog.
A review by the Medicines and Healthcare products Regulatory Agency (MHRA) found "no robust evidence" that many popular remedies work in children.
The MHRA said 36 medicines should no longer be sold for children under the age of six.
It has named eight cough mixtures that can be used for this age group.
In very rare cases some of the medicines can cause allergic reactions and hallucinations, the MHRA added.
But it stressed there were no specific safety concerns with the medicines it examined and parents could still give them to children over the age of six, if they felt they would benefit.
New, clearer dosage advice for children between six and 12 will also be published on packets.
Pharmacists will be issued with new advice to give to parents about which medicines to use.
But the MHRA said parents should not worry if they had used the medicines in the past, and shop shelves would not be cleared of current stocks.
Remedies used for pain relief and to lower a child's temperature, such as Calpol, are unaffected by the new rules.
MHRA spokesman Jeremy Mean said remedies proven to work on adults could not be assumed to have the same effect on children.
"Many years ago it was thought that we could use adult doses in a watered down way but we now know that children's bodies are different," he said.
He added that all children's medicines were being reviewed owing to a "change in thinking".
Dr June Raine from the MHRA said over-the-counter medicines had been used to treat colds for years but when they first emerged, the clinical trials were not required to demonstrate that they worked in children.
She told the BBC there was no reason for parents to be alarmed.
"What we've found is no major safety problem but a lack of evidence that these products actually do anything much to help a child," she said.
"So what we are advising parents is to stick to the simple, best practice - paracetamol, ibuprofen, warm lemon and honey. After all, a cough and a cold will get better on its own, after a few days."
The trade body for over-the-counter medicine manufacturers, the Proprietary Association of Great Britain (PAGB), said the affected remedies would no longer be marketed for children.
Executive director Sheila Kelly said: "PAGB and its member companies continue to have confidence in the value that these medicines have in the management of children's coughs and colds for children over six years.
"Products continue to be available for this older age group and manufacturers are committed to developing the robust evidence base that is now required."
High Street chemist Boots, which makes its own branded medicines, said it will be following the MHRA guidance.
The Royal Pharmaceutical Society of Great Britain, the professional and regulatory body for pharmacists in England, Scotland and Wales, said it considered it "good practice" to restrict some remedies for young children.
David Pruce, its director of policy, added that further research was needed on how effective over-the-counter medicines were for coughs and colds in children over six.