In some children Tamiflu caused nausea and vomiting, which can lead to dehydration and other complications, researchers reported.
The study did not cover the current outbreak of swine flu, but its conclusions suggest that antivirals may not significantly reduce the length of illness or prevent complications in children infected with the new A(H1N1) virus, the researchers said.
Carl Henegan, a doctor at John Radcliffe Hospital in Oxford and co-author of the study, said the current practice of giving Tamiflu for mild illness was "an inappropriate strategy."
"The downside of the harms outweigh the one-day reduction in symptomatic benefits," he said.
The research showed that antivirals oseltamivir and zanamivir shortened the duration of seasonal flu by up to a day and a half.
But the drugs had little or no effect on asthma flare-ups, increased ear infections or the need for antibiotics.
Tamiflu, the brand name for oseltamivir, was also linked to an increased risk of vomiting. Zanamivir is marketed under the name Relenza.
The study, published in the British Medical Journal, comes 10 days after Britain's Health Protection Agency (HPA) reported that more than half of 248 students given Tamiflu after a classmate fell ill with swine flu suffered side-effects such as nausea, insomnia and nightmares.
Most of the students did not have the flu when they were given the drug.