A guide to help doctors prescribe medicines for children has been published by child health experts.
Doctors treating sick children will be able to refer to the guide
The British National Formulary (BNF) for Children will help doctors treating newborns up to 18-year-olds.
Medics currently have to rely on their best judgement to decide what drugs and dosages to give to children, often based only on adult data.
Around 40% of medicines prescribed to anyone under the age of 18 have never actually been tested on children.
The guide is written by experts from the Royal College of Paediatrics and Child Health, the Neonatal and Paediatric Pharmacists Group, the British Medical Association and the Royal Pharmaceutical Society of Great Britain.
The government has committed £1.8m to fund the project, and 175,000 copies of the BNF will be provided free of charge for doctors and other prescribers of paediatric medicines.
It will be a companion to the BNF for adults, which doctors have used as their drugs "bible" for many years.
Its launch follows government's recent appointment of Dr Sheila Shribman as National Director for Children for the NHS in England.
Like the adult version, the BNF for Children be continuously revised, and a new edition will be produced each year.
Topics that will be covered include guidance on calculating paediatric doses and identifying and reporting adverse reactions to medicines in children.
Prescribers could also be given advice on prescribing medicines that are not licensed for children - so-called "off-label" prescribing.
The BNFC is part of a European initiative to develop more treatments for children and expand research and information in the field.
Dr George Rylance of the Royal College of Paediatrics and Child Health, who chaired the committee which put the children's BNF together, said: "When treating children, dosages must be tailored for each child, calculated by weight and age.
"Until now the information doctors needed to do this was not easily or readily available when they needed it - during a consultation for instance."
He added: "Most drugs for everyday children's ailments are licensed for paediatric use.
"But drugs developed for adults are not always tested for use in children.
"For complicated or more serious conditions, doctors often need to use drugs tested in adults which have not been specifically licensed for children (off-label prescribing), or which are not routinely available (unlicensed prescribing).
"The new guide will bring all this information together in one reliable source. It's a great step forward for children's medicine.
"For the first time, all doctors will have the latest advice at their fingertips."
Health Minister Jane Kennedy said: "The new BNF for Children is a very exciting development and an important tool in improving the quality of prescribing medicines for children."
Dr Shribman, added: "It's an excellent step that prescribers of children's medicines will now have the vital guidance that the BNFC provides."
Andrew Proctor of Action Medical Research, which has been campaigning for children to be treated differently to adults, said, "Babies and children differ from adults in the way they absorb and metabolise drugs, so scaling down adult dosages is not always safe or effective. "We are delighted to see that, finally, doctors can now refer to an authoritative BNF that puts the needs of children first - giving practitioners greater confidence when prescribing medicines. "While this is excellent news for paediatric care, it is also vitally important that work to test more drugs for use in children continues."