Clinical trials funded by the drug industry and other for-profit organisations are more likely to report positive findings than those funded by non-profit groups, according to a study published in the Journal of the American Medical Association.

To examine the issue, the authors of the study reviewed 324 consecutive therapeutic comparative trials involving cardiovascular therapies published in three high-impact medical – JAMA itself, The Lancet and the New England Journal of Medicine - journals between 2000 and 2005.

The researchers, Dr Paul Ridker and Dr Jose Torres from the Brigham & Woman’s Hospital at Harvard University, found that for non-profit organisation sponsors, 49% of the trials favoured new treatments over the current standard of care, while 51% did not. In contrast, 68% of trials funded by for-profit groups showed superiority of the new treatment, and for jointly sponsored trials 57% came out in favour of the new therapy, roughly midway between the two groups.

Interestingly, regardless of funding source, trials using surrogate end points, such as quantitative angiography, intravascular ultrasound, plasma biomarkers, and functional measures, were more likely to report positive findings (67%) than trials using clinical endpoints.

The findings reinforce the conclusions of similar analyses conducted in the 1990s which also found that for-profit organisation-sponsored trials were more likely to have a favourable outcome.

"As suggested in surveys of randomised trials published prior to 2000, these contemporary data appear to show that incentives surrounding for-profit organisations have the potential to influence clinical trial outcomes. Previous attempts to explain this phenomenon have focused largely on design bias, interpretation bias, data suppression, and differential data quality," said Ridker and Torres.

They noted that there are some ways to explain the results that do not necessarily mean the for-profit sponsors are influencing studies to their own ends. For example, they note that if a first trial of a new drug is positive, there is a strong likelihood that other trials will follow, and these are more likely to be sponsored by a for-profit group.

The drug industry has also come under considerable criticism for what is perceived as a lack of transparency in clinical trial disclosure. This in turn has led to accusations that companies have selectively published positive trial results whilst hiding negative ones, and a number of drugmakers – including GlaxoSmithKline, Merck & Co, Eli Lilly and Roche - have responded by adopting a policy of full disclosure, publishing details of all the trials they undertake.

In September 2004, the International Committee of Medical Journal Editors said it would stop publishing trial results unless the data were included on a public database.