Children should not be given the anti-viral drug Tamiflu to combat swine flu, Oxford University researchers said today.
They urged the Department of Health to urgently rethink its policy on giving the drugs to youngsters affected by the current flu pandemic.
Some 300,000 people in England, including children and adults, have received courses of Tamiflu through the Government's National Pandemic Flu Service for England.
Today's study, published in the British Medical Journal (BMJ), warned that Tamiflu can cause vomiting in some children, which can lead to dehydration and the need for hospital treatment.
The researchers said children should not be given the drug if they have a mild form of the illness although they urged parents and GPs to remain vigilant for signs of complications.
Parents of children with a compromised immune system or a condition like cystic fibrosis should discuss the harms and benefits with their GP, they said.
But overall, the researchers said, children who were otherwise healthy could suffer more harm than benefit from taking Tamiflu or another anti-viral, Relenza.
They found the drugs had little or no effect on asthma flare-ups, ear infections or the likelihood of a youngster needing antibiotics.
The researchers also found that using anti-virals preventatively had little effect - reducing transmission of flu by 8 per cent.
This means 13 children would have to be treated to prevent one additional case of the flu.
However, anti-virals could reduce symptoms by between half a day and one day.
Dr Carl Heneghan, a GP and clinical lecturer at Oxford University, said the current policy of giving Tamiflu for mild illness was an "inappropriate strategy".
He added: "The downside of the harms outweigh the one-day reduction in symptomatic benefits."
He and Dr Matthew Thompson, a GP and senior clinical scientist at Oxford University, analysed four studies involving children aged one to 12.
The children were being treated for normal seasonal flu but Dr Thompson said the findings would extend to the current swine flu pandemic.
"I don't think we have got any reason to think our results would be any different," he said.
"The current swine flu is generally a mild flu illness...it does not seem that different from current seasonal flu.
"We would be happy to say our results apply to the current swine flu strain."
He said children with mild symptoms should be treated in the same way as if they had any other mild flu - with drinks to cool high temperatures and rest.
Dr Heneghan said the only benefit found in the study was that children were back to normal half a day to one day earlier if taking Tamiflu or Relenza.
He said his advice to GPs was "not to rely on Tamiflu as a treatment to reduce complications" or to think of it as a "magic bullet".
And he warned that widespread use of Tamiflu could result in the flu becoming resistant to the drug.
"What is a problem going forward - like with antibiotics - is you run into a resistance issue.
"Going forward we have a treatment which is ineffective because we've given it to everybody."
Both researchers called on the Department of Health to review its current policy.
Dr Thompson said: "It's possible a more conservative strategy (such as) reserving these anti-viral drugs for people, for children who are more likely to have complications of the illness might be a more sensible strategy."
Dr Heneghan added: "I think the Government should be looking at this urgently, this week."
The experts said the studies had been publicly available to the Government before it formulated its current strategy with regard to Tamiflu.
And they said the Government should have demanded more data from the pharmaceutical companies which manufacture the drugs - Roche, which makes Tamiflu, and GlaxoSmithKline, which makes Relenza.
Today's research was published in the British Medical Journal (BMJ) and follows two recent studies which found that more than half of children taking Tamiflu suffered side-effects such as nausea, insomnia and nightmares.
Experts from the Health Protection Agency (HPA) found a high proportion of British schoolchildren reporting problems after taking the anti-viral preventatively.
The experts behind one of the studies said that although children may have attributed symptoms which were due to other illnesses to the use of Tamiflu, that was "unlikely to account for all the symptoms experienced".
That study was carried out in April and May - before the Government decided to stop using Tamiflu preventatively.
Only those with suspected or confirmed swine flu are now getting the drug and are being urged to get access to Tamiflu through the Pandemic Flu Service, which is accessed online or via a telephone helpline.
Children are known to be at high risk of catching the flu, with more than 40% of pre-school children getting the virus and 30% of school-age children doing so, the researchers said.
A Department of Health spokesman dismissed the researchers' claims that their findings would also apply to swine flu.
"The BMJ review is based on seasonal flu and not swine flu," he said.
"As the authors note, the extent to which the findings can be applied to the current pandemic is questionable - after all, we already know that swine flu behaves differently to seasonal flu, and past pandemics have hit younger people hardest.
"Whilst there is doubt about how swine flu affects children, we believe a safety-first approach of offering anti-virals to everyone remains a sensible and responsible way forward."
He said the policy would be kept under review and people with mild symptoms "may find bed rest and over-the-counter flu remedies work for them.
"But for those who experience severe symptoms, the best scientific advice tells us that Tamiflu should still be taken as soon as possible - and to suggest otherwise is potentially dangerous.
"If people are in any doubt about whether to take Tamiflu, they should contact their GP."
Liberal Democrat health spokesman Norman Lamb said: "This analysis needs to be taken extremely seriously and demands an immediate response from the Government.
"An urgent review must be carried out into whether the benefits of prescribing certain anti-viral drugs are worth the risks when it comes to our children's health."
A total of 36 people in England have died after getting swine flu.