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Clinical trials that compare two similar drugs are significantly more likely to favour the one made by the company that pays for the work, according to a study that sheds new light on bias in medical research.

An analysis of almost 200 published comparison trials indicated that when the findings show one drug to be superior, the study is about 20 times more likely to have been funded by its manufacturer.

The link between the funding and the outcome is even stronger when authors discuss the implications of their results: papers that include positive interpretations are about 35 times more likely to have been paid for by the drug’s maker.

The work, by a team led by Lisa Bero, Professor of Clinical Pharmacy at the University of California, San Francisco, raises fresh concern about the influence of pharmaceutical companies over research. Peer-reviewed medical literature is one of the sources used by funding bodies such as the National Institute for Health and Clinical Excellence and insurers when deciding which drugs should be made available.

“Many people are concerned about the growing proportion of drug trials funded by the drug’s manufacturers,” Professor Bero said. “If drug-trial outcomes are largely determined by who pays for the trial, we don’t really know what the best drug is.”

The research, published in Public Library of Science Medicine, examined studies of statins. It does not suggest that scientists funded by the drug industry manipulate data, but points to subtle ways in which they can be influenced, such as by comparing a normal dose with a low dose of a rival drug.

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