Around 400,000 people with cancer in the UK are lonely, with some skipping meals and unable to afford food, a charity has warned.
Macmillan Cancer Support's survey of more than 1,000 people with the disease found 22% suffered loneliness as a result of their diagnosis.
The charity said many were left housebound due to their cancer, causing isolation and knock on problems such as a poor diet.
The research also compared the experiences of cancer patients who said they were lonely with those who say they were not.
It found lonely people were three times more likely to admit drinking more alcohol than usual (22% compared to 7% for those who were not lonely).
They were also five times more likely to have not left the house for days (66% compared to 14%) and almost three times more likely to have problems sleeping (76% compared to 27%).
Lonely cancer patients were also five times more likely to skip meals (38% compared to 7%) and almost eight times more likely to eat a poor diet (45% compared to 6%).
Some 13% said they could not afford to buy food although other key reasons included having no food in the house, being too weak to cook and having no appetite.
People reported being particularly lonely if their cancer was advanced or had relapsed, they lived alone, or they had to make changes to their working life.
Ciaran Devane, chief executive of Macmillan Cancer Support, said: "Loneliness is blighting the lives of hundreds of thousands of cancer patients in the UK.
"It's hard enough for people being hit with the devastating news that they have cancer, without having to suffer the additional effects that being lonely brings. It's heartbreaking to think of people struggling to eat or leave the house because they have been abandoned and left to deal with cancer alone.
"This is a growing problem which is only set to get worse as the number of people diagnosed with cancer doubles from two to four million in the next 20 years."
Mr Devane said the NHS, policymakers and local authorities needed to "wake up to this looming loneliness epidemic" and work with the charity to ensure nobody faced cancer alone.
Clare Redgrove, 49, from Kent, was diagnosed with womb cancer in 2010.
She said: "Being diagnosed with cancer has been a very lonely experience. I went from being a busy person running my own business to living on benefits.
"As I live alone, there were days when I'd find it hard to find the energy to feed myself let alone get out of my house. I feel under tremendous stress, my sleep has suffered, and it all seems even worse now that my treatment is over."
Jack Neill-Hall, campaigns manager for the Campaign to End Loneliness, said: " When we feel lonely we can sometimes forget to look after ourselves properly.
"In fact, researchers have shown that feeling lonely can lead us to not exercising enough, eating a poor diet and having an increased likelihood of smoking and drinking too much. Unfortunately, these very behaviours can often contribute to us becoming ill, or aggravate other conditions outside of our control.
"It is vital that we understand the link between loneliness and ill health so that we can break the negative cycle of loneliness exacerbating ill-health and vice versa.
"By ensuring that our public health and care services are aware of the risks of loneliness, we can do more for people who may be suffering from cancer, disability, or duel sensory loss and make sure they are well looked after, but also better able to look after themselves."