Researchers found it was as dangerous as being a quarter over the legal drink-drive limit.
The study, published in the Traffic Injury Prevention journal, also found that having deep, thoughtful conversations while using a hands-free mobile phone was a danger.
In contrast, simple discussions may not carry significant driving risks, according to scientists from the University of Barcelona and several Australian institutes.
“Our results suggest that the use of hands-free devices could also put drivers at risk,” said Dr Sumie Leung Shuk Man, co-author of the study.