Objective To investigate potential violations of patient confidentiality or other breaches of medical ethics committed by physicians and medical students active on the social networking site Twitter.

Design Population-based cross-sectional observational study.

Setting The social networking site Twitter (Swedish-speaking users, n=298819).

Population Physicians and medical students (Swedish-speaking users, n=237) active on the social networking site Twitter between July 2007 and March 2012.

Main outcome measure Postings that reflect unprofessional behaviour and ethical breaches among physicians and medical students.

Results In all, 237 Twitter accounts were established as held by physicians and medical students and a total of 13 780 tweets were analysed by content. In all, 276 (1.9%) tweets were labelled as ‘unprofessional’. Among these, 26 (0.2%) tweets written by 15 (6.3%) physicians and medical students included information that could violate patient privacy. No information on the personal ID number or names was disclosed, but parts of the patient documentation or otherwise specific indicatory information on patients were found. Unprofessional tweets were more common among users writing under a pseudonym and among medical students.

Conclusions In this study of physicians and medical students on Twitter, we observed potential violations of patient privacy and other breaches of medical ethics. Our findings underline that every physician and medical student has to consider his or her presence on social networking sites. It remains to be investigated if the introduction of social networking site guidelines for medical professionals will improve awareness.