An international group of scientists has today raised further concerns about the lack of evidence supporting the safety and effectiveness of the blockbuster anti-flu drug Tamiflu.

They criticised the drug's Swiss-based manufacturer, Roche, for still not making available clinical trial data used to get the drug licensed. The company had promised to give the medical community access to the data following an investigation in December by Channel 4 News.

"We get the impression that the drug companies are being at best tardy," says Chris Del Mar from Bond University in Australia, who leads the Cochrane influenza review team. "Roche promised this information publically in December and we still don't have it."

During the swine flu pandemic last year courses of Tamiflu were handed out to anyone with symptoms calling the government's National Pandemic Flu Service. In the first two weeks of the pandemic, 500,000 courses of the drug were prescribed nationwide. The government spent more than £500 million stockpiling antiviral drugs like Tamiflu in preparation for a pandemic.

Last year, the Cochrane Collaboration, an international group of public health specialists found, during a routine review of flu drugs, that there was insufficient evidence in the public domain to account for Tamiflu's effectiveness as a pandemic flu drug. They particularly criticised a key paper used to justify Tamiflu's use during pandemics to international regulatory authorities.

"Roche promised this information publically in December and we still don't have it." Chris Del Mar, Bond University, Australia

In an interview with Channel 4 News in December 2009 Dr David Reddy, pandemic taskforce leader at Roche, said that to satisfy any concerns the medical community had, the company would make available "full study reports" from clinical trials carried out while bringing Tamiflu to market.

Earlier this year the Cochrane scientists were given access to a block of data about the drug but they say there is more.

"They promised us ‘full study reports' and they made available one - of what we think are ten study reports in existence," says Dr Tom Jefferson a member of the Cochrane review team based in Rome.

What's more, the data on Tamiflu that Roche did make available appears to raise serious questions about the data that is in the public domain.

For example, in the previously confidential trial data supplied by Roche to the Cochrane Collaboration "serious adverse events" were reported in ten patients. The data indicates the illnesses in three patients could have been caused by the drug itself. However in a subsequent publication about the drug - including one used by governments to support the stockpiling of the drug for Pandemics - it was declared the drug caused no serious side-effects.

While there is no evidence that the drug is harmful, there is a lack of available evidence to show otherwise, the Cochrane review team says. "The apparent contradictions in the data we have undermine the integrity of the data we have," says Jefferson.

In a statement today, Roche told Channel 4 News that it had already supplied 3200 pages of data to the Cochrane scientists. "Roche will consider providing additional information only once the Cochrane Group has provided us with its detailed analysis plan for review by our scientists and physicians."

In response to questions about serious adverse events appearing in trial data but missing from reports put into the public domain, the company said: "It is likely that the authors of the publication used their own medical judgment after examining the case in detail and arriving at their statement that "there were no drug-related serious adverse events".

International drug regulators, who approve medicines like Tamiflu should have been given access to all the clinical trial data on Tamiflu - including reports of any serious adverse events.

However in recent years a number of high-profile drugs have been later withdrawn despite having once cleared regulatory hurdles. Just last week, Avandia, a top-selling diabetes drug made by British company GSK was withdrawn from the market after regulators agreed with independent scientists that the medicine carried an unacceptably high risk of causing heart attacks.

The Cochrane group is also requesting more data from GSK which supported the approval of its anti-flu drug Relenza.