Because cancer drug trials are stopped early even when remote benefits are discerned, Italian researchers say the full effects of these drugs may have been exaggerated.
The researchers from the Mario Negri Institute for Pharmacological Research in Milan said that drugs hailed as breakthrough treatments for cancer may be less effective and cause more harm than suspected.
They added that data from many of the recent cases had been used to get drug licenses before the long-term impacts were known.
The research team conducted an analysis of 25 drug trials, including some for the breast cancer therapy Herceptin, that were stopped early over a ten year period between 1997 and 2007.
Their analysis showed that of the 25 trials randomly chosen, 14 had been stopped in the past three years, of these, 11 were used to support applications for marketing authorisation from regulators.
Dr Giovanni Aplone, lead author, said the increase in early conclusions to trials suggested drug firms were using good interim results to get their products to market more quickly.
"Data on effectiveness and potential side-effects can be missed by stopping a trial early," BBC quoted Aplone, as saying.
While Dr. Aplone admitted that hard evidence to back this claim is scant, he said an in-built bias exists in the system because trials were often only stopped early because the results were positive, when this could just be a 'random high', while, those that did not show such positive results were given more time to prove their worth.
The research team found that the average study duration was 30 months, when the long-term impact could only be judged over years and some trials only enrolled less than 40 percent of the total patients originally planned.
According to the researchers, regulators need to take into account the impact of stopping a trial early when making decisions about licenses and make more use of independent monitoring committees to verify trial data.
Presently only the largest trials adopt this approach.
Professor Stuart Pocock, an expert in medical statistics from the London School of Hygiene and Tropical Medicine, agree the issue is a problem, which is not confined to cancer drugs but all kinds of treatment.
He said there is a dilemma when results are positive because those patients involved in the studies, but not receiving the therapies, could lose out.
"We need proof beyond reasonable doubt to stop a trial early," he added.
A spokesman for the Association of the British Pharmaceutical Industry pointed out that the study analysed a small number of trials.
"When a trial is stopped early the data is carefully scrutinised and we are confident it is robust," he said.
"Patient safety is our top priority and when results are positive it can cost lives by not getting the product to market," he added.
The study appears in the Annals of Oncology.