Experts have warned there could be potential bias among some researchers examining the efficacy of drugs after a new study found that more than half have financial ties to drug companies.
Pharmaceutical companies paid researchers for consultancy or advisory work, speeches, travel costs, honorariums and stock ownership, according to a new paper published in The British Medical Journal (BMJ).
The new paper examined whether there was a link between these financial ties and the outcome of the trial.
They found that where there was such a connection, studies were more likely to have positive results.
The US-based researchers looked at a sample of 195 drug effectiveness trials published in 2013 and sought out whether there were financial ties between drug manufacturers and the lead authors of the study.
More than half (58%) of principal investigators had financial ties to the drug industry.
And papers written by lead investigators with such links were more likely than others to report favourable results, the study found.
" Financial ties of principal investigators were independently associated with positive clinical trial results," they conclude.
"These findings may be suggestive of bias in the evidence base."
Dr Jacintha Sivarajah, head of medical affairs at the Association of the British Pharmaceutical Industry, said: "The UK clinical trials environment works differently than in the US.
"In the UK, funding to run clinical studies is made available to organisations including NHS Trusts and Universities - rather than individuals.
"There should be no undue influence from any industry, pharmaceutical or otherwise, in the clinical and financial decisions of a NHS institution.
"Calling for peer-reviewed studies that have benefited from industry funding, or where authors have financial ties to the pharmaceutical industry, to be 'interpreted with caution' fundamentally undermines the positive collaboration which has allowed medical research to flourish in this country.
"We are proud of the partnerships our members take part in to develop innovative new treatments for patients, which include new medicines for a whole range of diseases."