Having a close relationship with a doctor could put patients' health at risk, according to a new study.
An online survey revealed that blurring professional boundaries may affect oncologists' objectivity when it comes to treating cancer patients.
The study, which polled 338 oncologists under 40, showed that 60% felt if doctors were too empathetic then they might not be able to make objective decisions, researchers at the Brighton and Sussex Medical School (BSMS) said.
Fifty nine per cent found it difficult to be truthful about prognosis if they particularly liked a patient, the study said.
Despite this, about a quarter of respondents had treated family members and a third had treated friends, the research found.
Most allowed patients to use their first name and 60% allowed patients to hug or kiss them in greeting or saying goodbye, it said.
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Such behaviours are not always wrong but could be liable to misinterpretation in a highly charged clinical setting, a BSMS spokeswoman said.
Lesley Fallowfield, director of Sussex Health Outcomes Research and Education in Cancer (SHORE-C) and professor of psycho-oncology at BSMS, said: "Oncology can be a very emotionally challenging field in which to work.
"Cancer patients need their doctors to be warm, compassionate and caring but the relationship should not be confused with a social one.
"Burnout in young oncologists is high, at more than 30%, and often leads to them leaving the profession.
"Any blurring of professional boundaries can also play a part in the stress faced by oncologists in dealing with patients with a life-threatening illness."
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The research, published in The Lancet Oncology, found that the rise of mobile technology and social media could add to the blurring of boundaries, with 55% of doctors admitting they had given their personal mobile numbers to patients and 14% becoming Facebook friends with patients.
Professor Malcolm Reed, incoming dean of BSMS, said: "Although these results need replicating, they show a worrying trend.
"The General Medical Council (GMC) has produced guidance in which these newer risks to the maintenance of professional boundaries are made clear; they need to be promoted to students and young doctors."
Ms Fallowfield continued: "It is so important that patients have strong and trusting relationships with their doctors during what will most likely be a very emotional and difficult period.
"Young oncologists clearly understand that and, as our survey shows, often invest a lot of personal time and emotional energy into doing what they believe to be helpful.
"But to ensure that cancer patients receive the best and most objective treatment, oncologists must look after their own emotional well-being also and recognise that there needs to be a professional line over which one just shouldn't cross."