Some health experts are calling into question the safety and effectiveness of the widely used anti-flu drug Tamiflu, which the World Health Organization calls an "essential medicine."

The British Medical Journal said it is still waiting for manufacturer Roche to release its clinical trial data for review, two years after it promised to do so.

"Roche's refusal to provide full access to all its data leaves critical questions about how well the drug works unresolved," said a report by the Cochrane Collaboration, a non-profit organization that reviews health-care research.

Cochrane researchers set out to test Roche's claims that Tamiflu prevents complications from flu and reduces the number of people needing hospital treatment. They say they found inconsistencies in some published reports and possible under-reporting of side-effects, but they can't conduct a complete analysis until the drug maker provides all of its data, the BMJ said in a statement Tuesday.

Based on its evaluation so far, Cochrane questions Tamiflu's effectiveness and says its ability to prevent the spread of flu has not been demonstrated.

Tamiflu is the drug many governments around the world - including Canada's - have stockpiled in case of a flu pandemic, based on a recommendation from the WHO.

"Until more is known...health professionals, patients and other decision-makers need to reflect on the findings of this review before making any decision about the use of the drug."

The Canadian Medical Association Journal reported on Cochrane's early findings in February 2010, and said the Public Health Agency of Canada stands by its recommendation that Tamiflu be used in the case of an H1N1 pandemic.

Roche maintains it provided the Cochrane team with enough information to conduct their evaluation, according to the BMJ statement.

Roche is also the maker of Avastin, which Health Canada recently de-authorized for use in the treatment of breast cancer, though it is still approved to treat other types of cancers.