The Medical Defence Union is concerned about sites such as Facebook
Doctors are being warned not to respond to flirtatious approaches on social networking sites.
The Medical Defence Union, a legal body for doctors, said communicating via sites such as Facebook may be a breach of ethical responsibilities.
It issued the warning after a number of cases in which patients propositioned doctors after searching for their details on the internet.
Regulators agreed that medics should be careful.
The MDU also said doctors should refrain from making comments that could identify patients, amid concerns some may "let off steam" to colleagues via Facebook believing it was a private forum when security settings meant anybody could see what was being said.
The group said new guidance from the General Medical Council explicitly mentioned "internet chat forums" in its rules on confidentiality.
Last year, a group of dental nurses were reprimanded by bosses for establishing a social networking group called "I'm a dental nurse and I hate patients because..."
The MDU also highlighted cases where patients had attempted to pursue inappropriate relationships with doctors who had treated them.
Earlier this year Birmingham University trainee medics were told to block their Facebook profiles after they were pestered by patients they had treated during hospital placements.
'Professional footing'
Dr Emma Cuzner, a medico-legal adviser at the MDU, said: "Some doctors have told the MDU they feel it would be rude not to reply, if only to politely refuse, but given that this is not a professional route of communication any correspondence of this sort would clearly stray outside the doctor-patient relationship.
"We are advising members about the importance of keeping relationships with patients on a professional footing.
"This is in line with doctors' ethical duties not to pursue improper relationships with patients."
A spokeswoman for the British Medical Association said it was right to be cautious.
"Social networking sites are part of everyday life and are extremely popular.
"However, the BMA believes doctors must maintain their professional relationship with their patients and not do anything that puts this in jeopardy."
A GMC spokeswoman added: "Doctors must understand that when using these sites, their conduct must remain within the boundaries of the doctor-patient partnership."
