The last time your doctor gave you a prescription, it likely came with specific information on the correct and safe dosage to take, determined  from years of clinical trials. But when kids need certain prescription drugs, such as statins, morphine, anesthetic or the asthma drug prednisone, doctors sometimes have to guess how much to give them based on the child's weight.

The reason, described in a report published today in the Journal of the American Medical Association, is that about half  the prescription drugs commonly given to  children have no information on appropriate pediatric doses on their labels. 

Researchers from the U.S. Food and Drug Administration studied drugs listed in the Physicians' Desk Reference, the bible of FDA-approved drug labeling , and counted the number  of drugs that came with guidance  for use in children, including the appropriate dosage.   Of 461 drugs in the PDR that were for children, 231 had the adequate information doctors would need to prescribe the drug to a child.

For the 105 brand new drugs approved by FDA between 2002 and 2008 that could potentially be used in children, 43 had pediatric information on the labels.

"You can't get a product approved in adults without studying how it affects them first," said Dr. Dianne Murphy, one of the study's authors. "But children are routinely being given products that are not studied in them."

The study is part of an effort to increase awareness that prescription drugs are often not tailored to kids' unique biology. In April, Harvard researchers reported that four out of every five children hospitalized in the U.S. are treated with drugs that have never been tested in children and are FDA-approved only for adults.

For newborns, the picture is even grimmer.  About 90 percent of drugs have never been tested for use in infants.

Dr. Florence Bourgeois, the author of that study and an assistant professor of pediatrics at Harvard, said  that about 60 percent of common conditions such as asthma and lower respiratory infections occur in children, but only 12 percent of the clinical trials currently testing drugs look at how they work in children specifically.

So when kids need these drugs, their doctors usually just have to guess how much to give them, usually gauging it against their weight. But kids are not just little adults. The way the liver, kidneys and other organs work to metabolize drugs is far different in early life.

"In some instances, extrapolating adult drug dosage to children might be appropriate, but again, without specific trials to assess that, we simply don't know," Bourgeois said.

Experts said testing drugs in children is simply more complicated and costly for drug companies. Clinical trials of children have their own ethical issues and often the science used to study adults doesn't apply to children. For example, it's difficult or impossible to get infants and children to breathe into a spirometer to study their asthma.

But  alarming as it may seem to parents with sick kids, the situation used to be a lot worse. Dr. Daniel Frattarelli, chairman of the American Academy of Pediatrics' Committee on Drugs, said even though half of drugs have dosage information for children, it used to be that only about 20 percent did.

"There has never been a safer time to use medications in children than right now," Frattarelli said.

The improvements are largely the result of two laws, passed in the late 1990s and early 2000s. The Pediatric Research Equity Act allows the FDA to require drug companies to do additional testing of their products on  children if the drug is likely to be used widely in pediatrics. The Best Pharmaceuticals for Children Act gives incentives for drug companies to test their products on children, such as giving them an additional six months of marketing exclusivity.

Both laws are up for congressional reauthorization in 2012, and experts say they are essential to getting more information about appropriate dosage of drugs in children.

ABC News' Dr. Veronica Sikka contributed to this report.