Findings published in the journal British Medical Journal (BMJ) question whether governments worldwide were justified in spending millions on the drug during the 2009 swine flu pandemic.
Researchers at the Cochrane Collaboration and BMJ have long argued there weren't enough trials to prove Tamiflu works and say after getting access to internal trials conducted by its maker, Roche AG, they found Tamiflu reduces flu symptoms by about a half a day in adults but has little effect on children.
Compared to people taking a placebo pill, there was no proof that people taking Tamiflu were less likely to be hospitalised or suffer serious flu complications.
Editor of the BMJ Dr Fiona Godlee said on Thursday there was a lack of "compelling evidence" that Tamiflu would "reduce your risk of pneumonia or hospitalisation if you get influenza".
Swiss pharmaceutical Roche said they "fundamentally disagree" with the overall conclusions of the BMJ series and that more than 100 countries have approved Tamiflu worldwide.
The US spent more than 1.3 billion US dollars on its antiviral stockpile while Britain spent nearly 711 million US dollars on Tamiflu in its pandemic preparations, according to government documents cited by the Cochrane group and BMJ.
Public health agencies including the World Health Organisation and the US Centers for Disease Control and Prevention continue to recommend Tamiflu be used to treat flu and say it has proven helpful in treating patients with novel strains like bird flu.