Prescribing drugs that have been properly tested only on adults to young children holds potential dangers.
Children and babies can not only absorb more of the drug into their tissues, but suffer different and more intense side-effects.
Currently two in five drugs used to treat children have never been licensed for that purpose, while 65% of drugs used on babies are being used outside the terms of their licence - or are not licensed at all.
What this means is that doctors frequently have no official reference guide to calculate the correct dose and timing of medication, relying on their own experience, judgement and in some cases guesswork as to the right amount.
Government action
Action Research, a charity which funds a variety of medical research, has teamed up with the Consumers' Association to call for more regulatory action from government - and more trials of new and existing drugs involving children.
John Grounds, Action Research's director, said: "Many drugs used on children on a daily basis are not tested or licensed for this use.
"The situation is an unacceptable and potentially-fatal one which needs to be remedied, for the benefit of all children, parents and doctors."
Clara Mackay, from the Consumers' Association, said: "At the moment children are being denied the same rights as adults in the provision of properly tested drugs.
"The government needs to set out a strong position in Europe to ensure that guidelines on drug testing in children are followed across the EU."
The drug companies point out that they do fund research into the effects of drugs on children, although the relatively small numbers of patients involved can make these difficult to organise.
Action Research itself has funded some research projects, one of which is based at Queen's University Belfast.
Professor James McElnay, of the School of Pharmacy there, said: "Children are not just little adults. Drug handling by children, particularly young ones, can be quite different to that in adults due to immaturity of the liver and kidneys."
His work has managed to study the effects of certain drugs on a sample of 300 children.