Loneliness has been linked to cancer in men.
While it may sound far-fetched, social isolation has previously been associated with a higher risk of death among cancer patients.
Feeling the connection was "poorly understood", scientists from the University of Eastern Finland analysed 2,570 middle-aged men over around 20 years.
The men who were deemed to be lonely were about 10% more likely to develop cancer, the results show.
It has been suggested isolated people may be less motivated to exercise regularly, eat well or drink in moderation. Nevertheless, the Finnish results remained the same after adjusting for the men's lifestyle.
The participants who were single at the start of the study and later developed cancer were also more likely to die with the disease than their coupled-up counterparts.
Although it is unclear, loneliness may trigger stress that disrupts a person's hormones and immune system. Isolation could also cause inflammation that "pre-disposes to various types of cancer".
One in two people born after 1960 in the UK will statistically develop cancer at some point in their life.
While smoking, obesity and excessive alcohol intake have repeatedly been linked to the disease, research into "the effects of psychosocial factors, such as loneliness and social isolation, is scarce", the Finnish scientists wrote in the journal Psychiatry Research.
Read more: Secondhand smoke raises mouth cancer risk
To learn more, the team analysed over 2,000 men aged 42 to 61.
Of these, 649 (25%) were diagnosed with cancer over the next two decades, of whom 283 died during the follow-up period.
Loneliness was estimated via two scales that measured "subjective satisfaction with one's social life" and "the perceived discrepancy between desired and actual social contacts".
Watch: Alexa could help with loneliness
The results linked loneliness to any form of cancer after adjusting for factors like age, alcohol intake, physical activity level, smoking status and sleep quality.
"It has been estimated, on the basis of studies carried out in recent years, that loneliness could be as significant a health risk as smoking or (being) overweight," said study author Siiri-Liisi Kraav.
"Our findings support the idea that attention should be paid to this issue."
Read more: Mushrooms cut cancer risk
The cancer patients who were single, divorced or widowed lived on average 7,851 days post-diagnosis, compared to 8,533 days among those who were married or cohabiting with a partner – a statistically significant difference.
"Social support" may be important post-diagnosis to help a cancer patient "initiate treatment in accordance with the prescribed treatment protocol".
Nevertheless, "the mechanisms behind these associations remain unclear and are a topic for further research", wrote the scientists.
"Awareness of the health effects of loneliness is constantly increasing," said Kraav.
"Therefore, it is important to examine, in more detail, the mechanisms by which loneliness causes adverse health effects.
"This information would enable us to better alleviate loneliness and the harm caused by it, as well as to find optimal ways to target preventive measures."
Watch: UK government urged to tackle post-pandemic loneliness