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.
Richard Bacon, a member of the Committee of Public Accounts, commented: "The ability of doctors, researchers and patients to make informed decisions about treatments is being undermined.
"The full results of clinical trials are being routinely and legally withheld from doctors and researchers by the manufacturers of medicines."
He said it was "of extreme concern to this committee" that favourable evidence was more likely to be published.
The group of MPs called for data on "all trials on all uses of all treatments currently being prescribed" to be made available.
Transparency in clinical trials is a decades-old issue. In the past year, the AllTrials campaign has focused attention.
Dr Ben Goldacre, one of the founders of the campaign and author of Bad Pharma, told the BBC: "It is quite frankly ridiculous that you would accept that half of clinical trial data can be withheld."
He added: "Patients aren't getting the best treatment for them... we have no way of knowing what the best antidepressant is and that is doing harm."
"We need urgent action from government, regulators, the General Medical Council, and all professional bodies to ensure that the full methods and results of all trials on all currently used treatments are made available to doctors, researchers, and patients."
In response, the Association of the British Pharmaceutical Industry (ABPI) insisted it was a "strong advocate for transparency" and said it was "misleading" to suggest the industry was routinely withholding data.
The organisation's research, medical and innovation director, Bina Rawal, said: "In late 2013, an ABPI-commissioned study was published in a peer reviewed journal.
"This study highlighted a positive trend of increasing levels of disclosure for industry-sponsored clinical trials, with almost nine out of 10 of all industry-sponsored clinical trials disclosed by 31 January 2013.
"However we recognise that there is still work to be done and we are continuing on a journey to achieving greater clinical trial transparency."
The report also raised questions about the UK government's half-a-billion pound stockpile of antiviral drugs in case of an influenza pandemic.
It said the decision was made "despite there being only limited evidence and widespread disagreement among regulators".
The manufacturers, Roche, handed over the full set of trial data last year and the analysis is still taking place.
In a statement the Department of Health said: "The UK is recognised by the World Health Organization as being one of the best prepared countries in the world for a potential flu pandemic and our stockpile of antivirals is a key part of being prepared."
On trials overall, it added: "We agree that clinical trials need to be more open and transparent and we're working with health organisations in the UK and EU to make this happen.
"All clinical trials conducted in the UK must now be registered on a public database."