The finding, from an evaluation of over 500 drug labels in 2009, is an improvement over the one in five labels that had pediatric information in 1975 -- but it still leaves a lot of questions, researchers said.

Often, drugs make it through the FDA approval process and to the market without much research on how they should best be used in kids, according to Dr. Eric Lavonas, from the Rocky Mountain Poison and Drug Center in Denver.

"Realistically, doing pediatric studies is hard," said Lavonas, who wasn't involved in the new report.

"The number of children available with a disease is usually less, and there are very real challenges to doing research on therapy for children ethically."

But lack of evidence often leaves doctors and parents "stuck" when it comes to treating kids, he told Reuters Health.

Legislation passed in the last decade allows the FDA to give some companies extra incentives to include pediatric information on their labels -- and the agency can require others to do the studies necessary to get that information.

FDA can also refer certain drugs to the National Institutes of Health for independent research on their uses in kids.

To gauge the effect of that legislation so far, Dr. M. Dianne Murphy and her colleagues at FDA analyzed the labels of 560 different medications used to treat everything from high blood pressure to allergies.

The labels were considered "adequate" to guide use in kids if they included information on how effective the drug is and whether it's safe in kids and teens, as well as the best dose for kids of different ages.

In their general analysis, the researchers found that 231 of the drugs were properly labeled, and another 29 had dosing information for at least some younger age groups, but not all.

That left 54 percent of labels without any useful pediatric information.

When the researchers removed the 100 medications that weren't relevant for use in children, such as prostate cancer treatments, 44 percent of the remaining drugs still had no kid-relevant labeling.

And more than half of drugs with new ingredients approved between 2002 and 2008 also didn't have specific information for safety and dosing in kids, the study team reported in the Journal of the American Medical Association on Tuesday.

Murphy said drugs that are frequently used in children -- such asthma medications and vaccines -- are often rigorously tested in that age group.

It's for the treatment of less-common pediatric conditions such as cancer and heart disease that those studies -- and evidence on efficacy, safety and dosing -- are lacking.

"Kids are mostly healthy but they still get heart disease, they still get lung disease, they still get endocrine disease, they still get arthritis, they still get many of the diseases that adults get -- and it's in those populations that we can't get things studied," Murphy told Reuters Health.

"Often we're stuck adjusting an adult dose to the size of the child without any real data to show whether that's the ideal dose," Lavonas said.

It's difficult to know if or how often that uncertainty has real consequences for kids, he added.

"If you give a child a therapy and it doesn't work, it's hard to tell if your dose wasn't ideal or it's just because the drug didn't work. Likewise if a child has a side effect, it's hard to tell if that's a side effect of the drug or if the child wouldn't have had that side effect if the right dose was given."

SOURCE: bit.ly/hwxtTL Journal of the American Medical Association, online May 8, 2012.

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