New York, Nov 18 (IANS): Having more than 300 Facebook friends may increase a teenager's levels of cortisol, the stress hormone, says a new study.

"While other important external factors are also responsible, we estimated that the isolated effect of Facebook on cortisol was around eight percent," said lead researcher Sonia Lupien, professor at University of Montreal in Canada.

"We were able to show that beyond 300 Facebook friends, adolescents showed higher cortisol levels; we can therefore imagine that those who have 1,000 or 2,000 friends on Facebook may be subjected to even greater stress," Lupien noted.

On the other hand, the researchers found that teenagers who act in ways that support their Facebook friends - for example, by liking what they posted or sending them words of encouragement - decreased their levels of cortisol.

Lupien and her colleagues recruited 88 participants aged 12-17 years who were asked about their frequency of use of Facebook, their number of friends on the social media site, their self-promoting behaviour, and finally, the supporting behaviour they displayed towards their friends.

Along with these four measures, the team collected cortisol samples of the participating adolescents. The samples were taken four times a day for three days.

"We did not observe depression in our participants. However, adolescents who present high stress hormone levels do not become depressed immediately; it can occur later on," Lupien cautioned.

The findings were published in the journal Psychoneuroendocrinology.