Social networking sites could be harming people's health by reducing levels of face-to-face contact, an expert has claimed.
Sites such as Facebook set out to enrich social lives, but end up keeping people apart, according to Dr Aric Sigman.
Lack of "real" social networking, involving face-to-face interaction, may have wide-ranging biological effects, he argues.
Evidence suggests that it could alter the way genes work, upset immune responses, hormone levels, and the function of arteries, and influence mental performance, says Dr Sigman.
This could increase the risk of health problems as serious as cancer, strokes, heart disease, and dementia.
Dr Sigman spells out his warning in the latest issue of Biologist, the journal of the Institute of Biology.
He maintains that social networking sites have played a significant role in people becoming more isolated.