The effects of antiviral drug Tamiflu - which was stockpiled by the Government at the height of the swine flu pandemic - are still uncertain, researchers said as they accused pharmaceutical giant Roche of withholding data.
A 2009 study in the British Medical Journal (BMJ) found the drug may cut the length of time people have symptoms but found no clear evidence that it prevented complications like pneumonia.
Experts from the Cochrane Collaboration conducted a review of 20 existing studies.
They found Tamiflu "did not reduce influenza-related lower respiratory tract complications" but did induce nausea.
Evidence of other adverse reactions to the drug were "possibly under-reported", they said.
The study criticised the evidence available from Roche and said the firm had not been able "unconditionally" to provide the information needed for the study.
At the height of the swine flu pandemic, the UK Government had contracts in place for 50 million doses of Tamiflu - enough for 80% of the population.
Tamiflu is still available for use when flu is circulating at high levels.
Today, a new report from the Cochrane Collaboration said the question of how well Tamiflu works remains unresolved owing to Roche's continued refusal to provide full access to its data.
Following the 2009 study, Roche did provide 3,195 pages of study reports for 10 treatment trials of Tamiflu but these were incomplete, the experts said.
"We cannot be certain whether the trial evidence supports the conclusion that (Tamiflu) reduces complications," they added.
"Nor do we understand if and how (Tamiflu) changes immune response, and important questions about its mode of action remain.
"But we think a critical analysis of the full clinical study reports - comprising tens of thousands of pages we have not yet seen - may provide critical answers."
The team added that "open access to all relevant trial data is a necessity to make ethical decisions in healthcare".
Dr Fiona Godlee, editor-in-chief of the BMJ, which ran an accompanying investigation about access to drug data, said "we are still a long way away from having a full trial history for all drugs in clinical use".
"Public safety and the proper use of public money demands that we should stop at nothing less than this."
Dr Peter Doshi from Johns Hopkins University School of Medicine, who worked on the new study, added: "In the BMJ in December 2009, Roche promised full study reports to any legitimate investigators.
"They have not provided a single full study report to Cochrane, despite our repeated requests."
A statement from Roche said: "Roche has made full clinical study data available to health authorities around the world for their review as part of the licensing process.
"It is the role of global health authorities to review detailed information on medicines when assessing benefit/risk.
"All completed Roche-sponsored clinical studies on the safety and efficacy of Tamiflu are available as peer-reviewed publications or in summary form on www.roche-trials.com.
"More detailed clinical trial reports are available for use by investigators on a password-protected site, enabling researchers to verify the findings of these studies and publications relating to them.
"Roche provided the Cochrane group with access to 3,200 pages of very detailed information, enabling their questions to be answered.
"Roche stands behind the robustness and integrity of our data supporting the efficacy and safety of Tamiflu."
A spokeswoman for the Department of Health said: "At the end of last year we published a major review of the scientific evidence base, which showed a clear benefit in offering antivirals to those with severe illness and those with additional risk factors."