The number of children taking the controversial drug Ritalin is on the increase, official figures suggest.
Ritalin is used to treat hyperactive children
According to the Department of Health, the number of prescriptions for the drug increased by 22% last year.
A total of 254,000 doses were prescribed last year, twice the amount given out five years ago.
Ritalin is prescribed to children with attention deficit hyperactivity disorder (ADHD).
The symptoms of ADHD range from poor concentration and extreme hyperactivity to interrupting and intruding on other people and not being able to wait in queues.
Studies have suggested the condition may affect one in 20 children. Boys are three times more likely to be diagnosed with ADHD than girls.
Ritalin is a mild stimulant - an amphetamine - that works on the central nervous system to improve concentration.
Studies have suggested the drug is an effective way of treating ADHD.
However, its critics say it can cause serious side-effects in some children, leaving them robotic, lethargic, depressed, or withdrawn.
The government's drug watchdog ruled in 2000 that Ritalin should be prescribed on the NHS to children with serious hyperactivity problems.
It is not licensed for people under the age of six, but doctors have prescribed it to children as young as 15 months.
There is also concern that doctors are prescribing the drug without considering alternative treatments.
Liberal Democrat health spokesman Paul Burstow has urged ministers to examine why the number of prescriptions has increased so sharply.
"There is a very big debate about the rights and wrongs of Ritalin but we need to look at why the prescription rates have gone up so steeply.
"It has become the option of first choice rather than a last resort for some families, but it needs to be given appropriately and only if it is really necessary," he said.
"There are still questions over the implications of giving such a powerful and illicit drug to very young children."