There has been a dramatic rise in the number of children on anti-obesity drugs but most stop taking them before they have any benefit, research found today.
Since 1999, the number of under-18s on the medicines has risen 15-fold across the UK, data showed.
Researchers estimated that 1,300 young people could now be being prescribed anti-obesity drugs every year.
The drugs are only licensed for use in adults, meaning they are being prescribed off-licence by family doctors.
The study, published in the British Journal of Clinical Pharmacology, looked at the use of orlistat (Xenical), sibutramine (Reductil) and rimonabant (Acomplia) in children up to the age of 18.
It focused on prescribing data from the UK General Practice Research Database between January 1 1999, and December 31 2006.
Overall, 452 youngsters received 1,334 prescriptions during the study period, and the prevalence of the drugs rose 15-fold among both boys and girls.
Most prescriptions were for 14-year-olds, although 25 prescriptions were written for children under the age of 12.
Orlistat accounted for 78.4% of all prescriptions and only one patient was prescribed rimonabant.
Around 45% of the youngsters stopped taking orlistat after only one month, as did 25% of those on sibutramine.
The estimated average length of treatment with orlistat was three months and four months with sibutramine.
The researchers, from University College London, said it was unclear whether this was because of side-effects, which include diarrhoea, or other reasons.
Russell Viner, one of the authors of the study, said: "It's possible that the drugs are being given inappropriately, or that they have excessive side effects that make young people discontinue their use.
"On the other hand, they could be expecting the drugs to deliver a miracle 'quick fix' and stop using them when sudden, rapid weight loss does not occur."
Guidance from the National Institute for Health and Clinical Excellence (Nice), published in December 2006, said the drugs should only be considered after dietary, exercise and behavioural approaches have failed.
"Drug treatment is not generally recommended for children younger than 12 years," the guidance said.
"In children aged 12 years and older, treatment with orlistat or sibutramine is recommended only if physical comorbidities (such as orthopaedic problems or sleep apnoea) or severe psychological comorbidities are present."
Study author Ian Wong said children who are prescribed orlistat may need more support and should be made fully aware of the potential side effects, which include loose, oily stools if fat intake is not reduced.
He said: "The key thing is that the drug itself is not the answer. Kids should only be using it as part of a comprehensive weight-loss programme."
According to the researchers, it is highly unlikely that the drugs are doing any damage, as short-term clinical trials have not found life-threatening side-effects in young people.
"Prescribing of unlicensed anti-obesity drugs in children and adolescents has dramatically increased in the past eight years," they said.
"The majority are rapidly discontinued before patients can see weight benefit, suggesting they are poorly tolerated or poorly efficacious when used in the general population.
"Further research into the effectiveness and safety of anti-obesity drugs in clinical populations of children and adolescents is needed."
Orlistat is now available over-the-counter in a half-strength dose (marketed as Alli). Alli is only available for obese adults aged 18 and over.
Tam Fry, from the National Obesity Forum, said: "It seems to me that we are ignoring measures to prevent our children becoming obese and then turning to drugs as a treatment of choice when they should be a last resort.
"That borders on criminal because it means that all the messages about healthy eating and exercise for reducing weight are just being tossed aside by GPs and primary care."
A spokeswoman for pharmaceutical company Roche said Xenical: "As always, we would suggest that if people are concerned about their children being overweight, they go and have a discussion with their GP," she said.
A Department of Health spokesman said: "People should where possible try to lose weight through a healthy diet and exercise before medical treatment is considered.
"However, for people who are obese and are not able to lose weight through diet and exercise alone anti-obesity medicines can help as part of an overall weight management programme."
Before any medicine is authorised for public use, clinical trials are carried out on adults to test the product's safety.
But the same is often not true for medicines used to treat children, because clinical trials are rarely conducted on youngsters.
Estimates are that more than half of medicines used by children have never been tested on younger age groups.Reuse content