The lack of data on the effectiveness of medicines available to doctors and researchers is "of extreme concern" say a group of MPs.
The Public Accounts Committee is calling for all data on drugs being prescribed in the UK to be made available.
It also says the government spent £424m stockpiling the antiviral Tamiflu despite a lack of agreement on how effective the drug is.
Campaigners called for "urgent action".
Half of trial data is not being published and pharmaceutical companies have a strong bias towards revealing the positive results, the committee's report says.
It raises questions about whether patients are being given the medication which is most likely to save their lives.
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