If we are in pain, we do something about it. We take painkillers, moan to a friend or go to a doctor. But what about those who can only express a reaction to pain with a grimace or tears? According to latest thinking on the subject, children, so often thought to be less susceptible to pain than adults, may be more sensitive to pain and its treatments than doctors had previously thought.

"The big advantage with adults is that doctors can ask them about their pain and they can then describe it. With children, the use of language is not so developed. They don't have these ways of explaining their pain," explains Dr Ann Goldman, specialist in children with chronic pain at Great Ormond Street Hospital. She is one of the organisers of the International Symposium on Children's Pain, which last week attracted 450 specialists to the Institute of Child Health in London.

"The whole medical fraternity has now realised that children are different," says Richard Howard, consultant in anaesthetics and pain management at Great Ormond Street, and, with Goldman, a director of the Children Nationwide Pain Research Centre. "In the past 10 years or so, neuro-biological research has shown that children, particularly newborn babies, experience pain in a different way to adults."

At Great Ormond Street, one innovation is to use a scale of faces, from smiling to distressed, to help young patients express the degree of pain. "It's more sophisticated than it seems," says Goldman. "We have to ensure that the child understands it in the way we think they do."

Specialists no longer assume that all humans feel pain in the same way. Research released in February this year by Jeffrey Mogil, professor of psychology at the University of Illinois, concluded that men and women experience pain differently. Could drugs aimed specifically at infants be the next step?

Goldman thinks not. "We'll be using the same drugs as for adults, but in a different way," she says. "In the past, children were not given as many painkilling drugs, proportionately, as adults for the same operations. If anything, it seems that they might need more." Young babies, in particular, can suffer: their developing central nervous system can often overreact to painful intervention, such as injections.

The same problems, however, apply to the administering of drugs to treat such pain. In babies, the enzymes in the liver that metabolise drugs are not fully developed. Incidences of mistakes in the treatment of young patients have highlighted the problems of using standard, adult, drugs to treat tiny bodies. In April last year, a junior doctor was acquitted of serious professional misconduct after miscalculating a morphine dose that resulted in the death of a premature baby.

Conversely, a teenage patient can present the opposite problem: drugs can be dealt with by the body more efficiently and make their way out of the bloodstream more quickly, which means that doctors may need to increase the level of analgesics.

Howard also challenges the assumption that very young children have no memory of pain, citing research in Canada - where, unlike in Britain, babies are routinely circumcised without anaesthetic - in which groups of circumcised and non-circumcised babies were tested for their reactions when receiving vaccinations. Those who had been circumcised displayed much stronger reactions than those who had not. Proof, says Howard, that "the earlier experience had influenced their future reactions to pain".

Great Ormond Street is soon to start a study that will follow children who have been treated with painkilling drugs - their input will be crucial in understanding the effects of pain, both long and short-term. Scientists, doctors, anaesthetists, nurses and psychologists, as well as the parents of children suffering acute pain, will all have their say. "We've realised that we really need to be able to understand this," says Howard. "Pain in children sounds very emotive, but it's only now that we've been able to make it a top priority."