They found that, although the research methods were of comparable standards, studies sponsored by drug companies were more likely to have favourable outcome for that company's products and were also less likely to be published.

Writing in the British Medical Journal, which this week is devoted to the relationship between doctors and drug companies, the authors said their findings applied across a wide range of diseases and drugs during the last two decades. They said this suggested there was a "systematic bias" in the outcome of research funded by the pharmaceutical industry.

A failure to supply an appropriate comparator drug or dose of comparator drug may explain why the results of these trials are skewed in favour of the sponsors' products, the researchers say.

A second study, also published in the British Medical Journal, concludes that drug treatment is likely to be provided on the basis of biased evidence precisely because drug companies tend to publish studies with more favourable results.

The researchers, from the Medical Products Agency in Sweden, identified 42 short-term clinical trials submitted to the Swedish drug regulatory authority to secure marketing approval for five antidepressant drugs. These trials were then compared with studies published between 1983 and 1999.

The researchers found evidence of three sources of bias: duplicate publication, selective publication and selective reporting. For example, 21 trials contributed to at least two publications each and those trials showing significant effects of a drug were more likely to be published as stand-alone publications than those with non-significant results.

The researchers warn that their findings show that people using only published studies to choose a drug may have their decision influenced by pharmaceutical company bias.

© HMG Worldwide 2003