Loneliness may raise the risk of a heart attack by more than 40%.
A major study published this week also suggests that social isolation can increase the chance of a stroke by 39% and premature death by up to 50%.
The analysis is based on the health records of 480000 Britons, making it the largest study of its kind.
Those who already had cardiovascular problems were far more likely to die early if they were isolated, suggesting the importance of family and friends in aiding recovery.
The research team said lonely people had higher rates of chronic diseases and showed more symptoms of depression.
Christian Hakulinen, the University of Helsinki expert who led the study, concluded that having few social contacts was a risk factor for early death, particularly among those with pre-existing cardiovascular disease.
“If we target conventional risk factors then we could perhaps reduce cardiovascular disease among those who are isolated or lonely,” said Hakulinen. “It is also important we show that those who are socially isolated might have a worse prognosis after a heart attack or stroke.”