Family doctors are to be advised that oral cough and cold medicines should not be given to children under six years old.
Medsafe, which is part of the Ministry of Health, has been advised that the evidence of harm outweighs the limited evidence of effectiveness.
The agency says the only cough and cold medicines it considers suitable for young children are those containing the ingredient bromhexine.
Medsafe will work with manufacturers to get newly labelled medicines on pharmacy shelves by next year.
But to prevent a shortage of products for adults, medicines with old packaging will not be removed from sale.
In December 2007, the Medicines Adverse Reactions Committee ruled cough and cold medicines were too risky for children aged under two, due to uncertainty over safe dosages.
Medsafe has now extended that warning to children aged under six and has advised the Royal College of General Practitioners.