TUESDAY, Sept. 23 (HealthDay News) -- More than half of all supporting clinical trials for U.S. Food and Drug Administration-approved drugs remain unpublished five years after permission has been given to sell the drugs in the United States, say University of California, San Francisco researchers.

They combed the medical literature to determine the publication status of all 909 clinical trials that supported the 90 new drug applications approved by the FDA between 1998 and 2000.

They found that 76 percent of pivotal trials -- such as large Phase II and Phase III trials designed to determine the overall risks and benefits of a drug -- had been published in medical journals, usually within three years of FDA approval of the drug. However, only 43 percent of all supporting trials submitted to the FDA had been published.

The UCSF team also found evidence of selective reporting of the results from these trials. For example, a pivotal trial in which a new drug outperforms an old drug is more likely to be published than a trial showing a new drug is no better than an old one.

This type of publication bias may lead to an inaccurately favorable record in the medical literature of a drug's performance compared to similar drugs. That can cause physicians to favor newer and more expensive drugs, the researchers explained.

They said their study findings, published in the journal PLoS Medicine, provide a baseline for monitoring the impact of the FDA Amendments Act 2007. It was introduced to improve the accuracy and completeness of drug trial reporting.

The act requires that all trials supporting FDA-approved drugs be registered when they start and that all outcomes must be posted within a year of drug approval on the U.S. National Institutes of Health's clinical trials Web site.

More information

Here's where you can learn more about the FDA's drug approval application process.