Missing data from clinical drug trials distorts the evidence and stops patients and doctors making informed choices about treatment, an MP has argued.
Dr Sarah Wollaston, MP for Totnes, raised concerns in Parliament that not enough is being done to ensure such information is made public.
It comes after the former GP stressed the need for drug companies to publish the results of "vitally important" research.
Pharmaceutical firms have denied accusations that some do not release data showing medicine is less effective than expected. But Dr Wollaston said the Government had spent £500 million stockpiling Tamiflu despite companies "holding back" full clinical study reports about the drug's effectiveness.
She said: "You have to ask yourself why is that being held back. This is hugely important. And it's not just about wasting money. This very much matters.
"This really is a current issue. It affects patient safety and it's wasting millions. If we could see a release of all the historic data… I think we would have a completely different evidence base for medicine. I think it's vitally important for patient safety."
Responding to a question by Dr Wollaston in the House of Commons, the Government committed to support a new European register which will publicly disclose the results of clinical trials conducted during the development of new medicines.
Health minister Norman Lamb said the register would make it easier for doctors and their patients to review which medicines were successful amid concerns that pharmaceutical companies are withholding results.