Doctors questioned the effectiveness and safety of the main drug used to treat pandemic flu, in a report out Wednesday.
International experts said Tamiflu cuts the duration of symptoms, but they found no clear evidence that it reduced the number of patients needing hospital treatment.
They also claim that side effects from the drug were "possibly underreported."
Swiss-based Roche Pharmaceuticals denies the allegations.
But doctors at the Cochrane Collaboration assessed 16,000 pages of clinical trial data and said there were "critical questions about how well the drug works and about its reported safety profile."
Researcher Carl Heneghan said, "What you are looking at in any drug is, what are the benefits and what are the harms? Right now, without access to the evidence, we are not in a position to say the benefits clearly outweigh the harms."
The Cochrane researchers claim that data from 60 percent of people involved in phase III clinical trials was never published.
"We are concerned that these data remain unavailable for scrutiny by the scientific community," the doctors said.
But Roche insisted that 80 percent of clinical data is open to independent researchers and that it is working to make the rest publicly available.
"Roche stands behind the robustness and integrity of our data supporting the efficacy and safety of Tamiflu. Numerous clinical studies -- and real-life medical experience -- show Tamiflu is effective in reducing the severity and duration of symptoms," the company said in a statement.