This means that an estimated 250,000 people have been exposed to the risks of trial participation without the societal benefits that accompany the dissemination of their results, say the authors.

They argue that this "violates an ethical obligation that investigators have towards study participants" and call for additional safeguards "to ensure timely public dissemination of trial data."

Randomized clinical trials are a critical means of advancing medical knowledge. They depend on the willingness of people to expose themselves to risks, but the ethical justification for these risks is that society will eventually benefit from the knowledge gained from the trial.

But when trial data remain unpublished, the societal benefit that may have motivated someone to enrol in a study remains unrealized.

US law requires that many trials involving human participants be registered -- and their results posted -- on the largest clinical trial website ClinicalTrials.gov. But evidence suggests that this legislation has been largely ignored.

So a team of US-based researchers set out to estimate the frequency of non-publication of trial results and, among unpublished studies, the frequency with which results are unavailable in the ClinicalTrials.gov database.

They searched scientific literature databases and identified 585 trials with at least 500 participants that were registered with ClinicalTrials.gov and completed prior to January 2009. The average time between study completion and the final literature search (November 2012) was 60 months for unpublished trials.

Registry entries for unpublished trials were then reviewed to determine whether results for these studies were available in the ClinicalTrials.gov results database.

Of 585 registered trials, 171 (29%) remained unpublished. Of these, 133 (78%) had no results available in ClinicalTrials.gov. Non-publication was more common among trials that received industry funding (32%) than those that did not (18%).

"Our results add to existing work by showing that non-publication is an important problem even among large randomized trials," say the authors. Furthermore, the sponsors and investigators of these unpublished trials infrequently utilize the ClinicalTrials.gov results database.

The lack of availability of results from these trials "contributes to publication bias and also constitutes a failure to honor the ethical contract that is the basis for exposing study participants to the risks inherent in trial participation," they add. "Additional safeguards are needed to ensure timely public dissemination of trial data," they conclude.