A first research to track every prescription drug approved by the U.S. Food and Drug Administration (FDA) from 1998 to 2000 has concluded that many studies finding the medicines ineffective were not published in medical journals.

Associate professor of internal medicine at UCSF Ida Sim, and lead author of the analysis published in the Public Library of Science journal, PLoS Medicine, said there was a pattern “that favorable studies were more likely to be published than unfavorable trials (and) this is something that is essentially structural in the way clinical trial information is disseminated to the public.''

Companies must submit at least two studies to the FDA demonstrating a drug is safe and effective to win its approval, and even if other clinical trials show negative results.

The researchers tracked all studies submitted to the FDA by drug companies seeking approval for new treatments from 1998 to 2000. By 2005, five years after the last FDA approvals of the drugs, most of the studies finding the drugs ineffective hadn´t been published in journals. Other trials finding them effective were much more likely to be published.

Critics of the system have said in the past that drug companies selectively publish results that show when a drug works and fail to publish those which don´t and the researchers pointed out that failure to openly publish both results gave a biased view for both doctors and the public.

This publication bias led Congress to last year mandate the creation of a registry of clinical trials. Under that legislation, drug companies must submit the results of all trials they conduct for posting on a government website that lists trials and results.

Concern that publication bias -- the tendency for positive studies to be more frequently published than negative ones -- may mislead consumers led Congress last year to mandate the creation of a registry of clinical trials.

Vice president of the Pharmaceutical Research and Manufacturers Association Ken Johnson said his organization supported the new legislation as a way to gain “more comprehensive information about those trials.''

However, Sim said that while the new law may help ensure that studies finding the drugs ineffective aren´t buried, a brief summary of study results posted on a Website isn´t the same as full publication in a medical journal.

“Medical journals are one of the most influential ways that clinicians and the public get evidence about which drugs work or don´t,'' she said. That's partly because of the attention published studies attract from the media, she said.

The new law may give drug companies less motivation to submit studies to journals because they can argue the Web-site summaries are providing full disclosure.

Sim and her colleagues searched the electronic databases of journals looking for published results of 909 trials submitted to the FDA in support of 90 drugs that were later approved for sale. They found that 57 percent of the trials, 515 in all, were never written about in studies published.

The study didn´t examine whether trials were submitted and rejected by journal editors or simply weren't submitted at all. Sim said previous research has shown that the primary reason for publication bias is that companies or investigators do not submit them.