Experts have called for new rules governing the relationship between the medical profession and the pharmaceutical industry.
Some doctors may be prescribing unsuitable drugs
It follows claims that some drug companies may have undue influence over what medicines doctors prescribe to patients.
Several studies published in this week's British Medical Journal (BMJ) suggest that this influence may cause some doctors to prescribe unsuitable and unnecessary drugs.
They raise questions about the free gifts given by companies to doctors and the way the industry funds research, medical conferences and education seminars.
Drug company representatives visit thousands of doctors every week to inform them about new or existing products. They often offer doctors free gifts, such as stationery, mugs or key rings. Sometimes the gifts may be more substantial.
One of the studies in this week's BMJ suggests this may encourage some doctors to prescribe unsuitable drugs to patients.
Doctors must not be, or be seen to be, influenced in prescribing matters by any incentives from the pharmaceutical industry
Dr Vivienne Nathanson, BMA
Researchers carried out a survey of 1,000 GPs across England. They found that doctors who see company reps at least once a week are more likely to consider prescribing new drugs and to agree to patients' requests for medication even if they don't need it.
They suggested that in some cases reps may even target those doctors who are most likely to prescribe their products.
Doctors are expected to only prescribe new drugs if there is medical evidence to show they are effective.
The researchers said doctors should be given better guidelines to ensure they only prescribe such medicines.
A separate study, by researchers in Sweden, suggests that even doctors who rely on research to decide which drugs to prescribe may also be unwittingly colluding with the pharmaceutical industry.
The researchers said this was because studies into new drugs were sometimes biased. They suggested that industry-funded trials were in some cases only published if they included favourable results.
The UK-based pharmaceutical industry strives to maintain the highest possible ethical standards
And they said a lack of access to negative studies meant some doctors were probably prescribing drugs on the basis of biased research.
Their findings were backed up experts in Canada. They reviewed 30 studies analysing research projects, which had been funded by a pharmaceutical company.
They found that these studies were more likely to come up with results that favoured the company compared with those that received funding from other sectors.
But they also found that these studies were less likely to be published in medical journals.
Richard Smith, editor of the BMJ, said the studies showed that the relationship between the medical profession and industry needed to change.
"Our central argument is that doctors, drug companies and most importantly patients would all benefit from greater distance between doctors and drug companies," he said.
Ray Moynihan, a medical journalist and guest editor of this week's BMJ, said the findings highlighted an unhealthy relationship between the medical profession and the drugs industry.
"We hope this will spark a broad debate about how to clean up the unhealthy aspects of the relationships between doctors and drug companies," he said.
The Association of the British Pharmaceutical Industry (ABPI) said companies were expected to adhere to strict guidelines when informing doctors about new products. It said the onus was on doctors to ensure they prescribed effective medicines.
Dr Trevor Jones, the association's director general, added: "The UK-based pharmaceutical industry strives to maintain the highest possible ethical standards in its dealing with healthcare professionals and other stakeholders."
The British Medical Association (BMA) said it hoped the studies would lead to a debate about the best way to prevent drug companies from inappropriately influencing doctors.
Dr Vivienne Nathanson, the BMA's head of science and ethics, added: "The guidelines in the UK put fairly strict limits on hospitality, gifts, sponsorship but should be constantly reviewed especially against any evidence of abuse or ineffectiveness.
"The individual needs of the patient are paramount when a doctor is prescribing medication.
"Doctors must not be, or be seen to be, influenced in prescribing matters by any incentives from the pharmaceutical industry."