June 30 (Reuters) - The U.S. Food and Drug

Administration said on Monday it is revising the labeling of all

extended-release stimulants to treat attention-deficit

hyperactivity disorder (ADHD) to warn about the risk of weight

loss and other side effects in patients younger than six years.

The FDA said that it is requiring a "limitation of use"

section in the prescribing information of all extended-release

stimulants to include a statement on higher rates of adverse

reactions in children younger than six years.

ADHD is a disorder, common among children, that affects the

ability to pay attention, follow directions and complete tasks.

The FDA has approved two types of medications - stimulants

and non-stimulants - to help reduce symptoms of ADHD.

Stimulants, sold under brand names Ritalin by Novartis

and Concerta by Johnson & Johnson's ( JNJ ) unit

Janssen, are commonly prescribed by healthcare professionals.

They work by increasing levels of dopamine and norepinephrine in

the brain, two neurotransmitters associated with motivation,

attention and impulse control.

Extended-release stimulants are not approved for children

younger than six years, but healthcare professionals can

prescribe them "off label" to treat ADHD, the FDA said.

The agency said it assessed data from clinical trials of

extended-release formulations of amphetamine and methylphenidate

for ADHD treatment, finding that, compared to older children,

patients younger than six years have higher levels of the drug

in their bodies as well as higher rates of side effects.

For these reasons, the benefits of extended-release

stimulants may not outweigh the risks in patients younger than

six years, the FDA said in a statement.

Studies show that the number of children diagnosed with ADHD

continues to increase. An estimated seven million children in

the U.S. between three to 17 years of age have been diagnosed

with ADHD, according to the FDA.