Children under the age of 12 should not be given Tamiflu or Relenza, the two antiviral drugs that form the cornerstone of the government's fight against swine flu, because their side-effects outweigh any benefits, Oxford University researchers said yesterday.

A study by Dr Matthew Thompson, a clinical scientist, and Dr Carl Heneghan, a clinical lecturer, found that antivirals were no "magic bullet" and only led to a minor reduction in the length of time that a child with mild sickness was ill or the chance of their spreading the disease.

The research, to be published in the British Medical Journal, said that Tamiflu caused vomiting in 5% of children, which could lead to dehydration and complications.

The Oxford researchers, both GPs, 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 – and there was no need for children who were otherwise healthy to be taking Tamiflu or Relenza. They called on the government to be clear about the limitations of the two drugs.

"It would be quite reasonable for the Department of Health to make clear to parents and GPs how much help the drugs are," Thompson told a press conference.

The research, a peer review of seven trials, all but one previously published, looked at the effectiveness of antivirals in reducing seasonal flu complications and stopping the spread of the disease. They said their findings were relevant to the current swine flu outbreak because the diseases shared many characteristics.

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 preventive antiviral use had little effect, reducing transmission of flu by 8%. Thirteen children would have to be treated to prevent one additional case of the flu, they said. The benefit of antivirals was that they reduced symptoms.

Heneghan said the current policy of giving Tamiflu for mild illness was an "inappropriate strategy".

"The downside of the harms outweighs the one-day reduction in symptomatic benefits," Heneghan 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".

He likened the current policy, which saw more than 315,000 courses of antivirals given out during the first two weeks the national pandemic flu line was in operation, to misguided historical health policies such as the prescription of antibiotics for sore throats.

"The problem is a resistance issue," said Heneghan. "Going forward we have a treatment which is ineffective because we've given it to everybody. We are calling for a more rational prescription process for these drugs."

One of the country's most respected flu experts, Professor Hugh Pennington, agreed that reliance on antivirals was increasing the possibility that the flu would become resistant sooner or later.

"We cannot be certain that the virus will not get nastier. We might require antivirals for the elderly or others who are more likely to get complications. It means the potential that a resistant strain will get that selective advantage as it will grow in a Tamiflu environment."

Norman Lamb, the Liberal Democrat health spokesman, said: "The government antiviral policy and flu line need to be kept under constant review. The cost involved is very high. Perhaps it should come down and be relaunched in the autumn. These are judgments for the chief medical officer."

The Department of Health disputed the relevance of a seasonal flu study to the current outbreak. "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 antivirals to everyone remains a sensible and responsible way forward. However, we will keep this policy under review as we learn more about the virus and its effects."

Roche, which makes Tamiflu, said: "Health authorities worldwide such as the National Institute for Health and Clinical Excellence (Nice), European Medicines Agency (EMEA) and the US Food and Drug Administration (FDA) have reviewed the data from controlled clinical trials and approved Tamiflu (oseltamivir) for use in children based upon a positive benefit/risk assessment.

"There are significant data showing the medicine to be effective and well tolerated when used for the treatment or prevention of influenza in children."