Urgent safety studies are needed for newer anti-epilepsy drugs that are being increasingly prescribed for children, say UK researchers.
The research looked at drugs given to 8,000 children
The British Journal of Clinical Pharmacology report says prescriptions have risen five-fold in 13 years.
Yet the drugs' long-term safety has not been established, say the researchers.
Many medicines are not fully tested on children before licensing, meaning consultants have no official guidance on doses to refer to when prescribing.
Instead they often have to estimate a safe and effective dose based on the age and the size of the child.
Lead author Professor Ian Wong, from the Centre for Paediatric Pharmacy Research in London, studied antiepileptic drugs given to nearly 8,000 children over a 13-year period.
The centre is a collaborative project run by the School of Pharmacy at the University of London, the UCL Institute of Child Health and Great Ormond Street Hospital.
Professor Wong and his colleagues found three drugs in particular - lamotrigine, topiramate and levetiracetam - had seen a "massive" rise in prescribing.
Professor Wong said this was concerning.
"The uptake of these drugs has been rapid, yet their long-term safety has not been established and further research must now be seen as a priority."
He said that while newer drugs were less likely to react with other prescribed drugs, it was possible that restrictions in their use might have to be introduced as any side-effects in children became more apparent.
He said multi-organ failure, renal failure, blood disorders and skin reactions had been reported in patients using lamotrigine.
Dr Jeffrey Aronson, a reader in clinical pharmacology at Oxford University, said many drugs were not tested on children because of the difficulties that including children in clinical trials posed.
"This means that clinicians often have to rely on their experience and scale down adult drugs for paediatric use, which is widely regarded as most unsatisfactory," he said.
Dr Colin Ferrie, a consultant paediatric neurologist at Leeds General Infirmary, said that not only were new trials important, but pharmaceutical companies should be encouraged to carry out "clinically relevant" trials prior to the original granting of a licence.
"Obviously it's an area of concern, and when you are prescribing a drug 'off licence' to a child, it's important to let the family know exactly the implications of this.
"However, it is often possible to find data, either from journal abstracts or the drug company, which can help you calculate a safe and effective dose of a drug."