Experts say that's almost beside the point, because use is rising on both sides of the Atlantic. And with scant long-term safety data, it's likely the drugs are being over-prescribed for both U.S. and U.K. children, research suggests.
Among the most commonly used drugs were those to treat autism and hyperactivity.
In the U.K. study, anti-psychotics were prescribed for 595 children at a rate of less than four per 10,000 children in 1992. By 2005, 2,917 children were prescribed the drugs at a rate of seven per 10,000 -- a near-doubling, said lead author Fariz Rani, a researcher at the University of London's pharmacy school.
The study is being released Monday in the May edition of the journal Pediatrics.
By contrast, an earlier U.S. study found that nearly 45 American children out of 10,000 used the drugs in 2001 versus more than 23 per 10,000 in 1996.
There are big differences that could help explain the vastly higher U.S. rate.
A recent report in The Lancet suggested that the U.K.'s universal health care system limits prescribing practices there. The report also said direct-to-consumer ads are more common in the United States. These ads raise consumer awareness and demand for medication.
While drug company ties with doctors are common in both the U.S. and U.K., Vanderbilt University researcher Wayne Ray said U.K. physicians generally are more conservative about prescribing psychiatric drugs. Ray co-authored the U.S. study, published in 2004.
The new U.K. study, involving 1992-2005 health records of more than 16,000 children, is the first large examination of these drugs in U.K. children. It found the increase was mostly in medicines that haven't been officially approved for kids. They were most commonly prescribed for behavior and conduct disorders, which include attention deficit disorder.
Side effects including weight gain, nervous-system problems and heart trouble have been reported in children using these drugs and there's little long-term evidence about whether they're safe for them, the study authors said.
"This highlights the need for long-term safety investigations and ongoing clinical monitoring," they said, "particularly if the prescribing rate of these medicines continues to rise."
One of the most commonly used anti-psychotics in the U.K. study was Risperdal, a schizophrenia drug that is sometimes used to treat irritability and aggression in autism. Its side effects include drowsiness and weight gain.
Thioridazine, sometimes used to treat hyperactivity in attention deficit disorder, was frequently used early on. Its use decreased after 2000 when a U.K. safety committee warned of heart-related side effects, the authors said.
Reasons for the increases are uncertain but may be similar to those in the United States, such as an increase in autism cases and drug industry influence.
In both countries, the issue isn't simply how many children are getting these drugs, said Dr. David Fassler, a University of Vermont psychiatry professor. "The more important question is whether or not the right kids are getting the most appropriate and effective treatment possible," he said. Fassler wasn't involved in the study.
Dr. William Cooper, a Vanderbilt pediatrician, said the study shows the drugs are being used "without full understanding about the risks."
"I find it really interesting that we're now seeing increases in other countries besides the U.S., which suggests that the magnitude of this issue is global," said Cooper, also an author of the 2004 U.S. study.
On the Net: American Academy of Pediatrics: www.aap.org