Oxford University academics found that some clinical trial findings were simply not reported, while others changed tack once results had begun to emerge, failing to stick to the original aims set out in the research protocol.

Researchers analysed the published results of more than 100 clinical trials and found that almost two thirds of results relating to potentially harmful trial outcomes were not fully reported. Half of results relating to the effectiveness of the treatment on trial were not made fully public.

“The problem is really in things which are not being reported,” Douglas Altman, a researcher at the Institute of Health Studies, Oxford, said. “There is a tendency to withhold the less interesting stuff.”

Eighty-six percent of the clinical trial teams contacted about the findings denied the existence of unreported outcomes, despite clear evidence to the contrary.

Dr Altman said: “In an appreciable number of cases, there were discrepancies between what they said they would do and what they had actually done. Whether there is anything sinister in that is very hard to tell.”

The research team has called for proposed new trials to be registered, with aims and methodology made public.

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