Fears of adverse reactions have been raised over a flu vaccine prescribed for millions of Britons.
EU regulators said Pandemrix is not ideal for young people and that the drug should be prescribed to under-20s only if other therapies are not available and they still need immunity to the H1N1 strain of flu.
The warning from the European Medicines Agency (EMA) comes in the wake of reports from Finland and Sweden that children and adolescents were suffering with narcolepsy after being administered Pandemrix.
Studies have shown a six to 13-fold raised risk of narcolepsy in children and adolescents vaccinated with Pandemrix, compared with unvaccinated children, according to the EMA.
At the same time, no increased risk was found in people over the age of 20.
"The vaccine is likely to have interacted with genetic or environmental factors which might raise the risk of narcolepsy, and other factors may have contributed to the results," the EMA said.
Pandemrix, manufactured by GlaxoSmithKline, was the most widely used drug in Britain at the height of the 2009/10 flu pandemic.