One in six people living with cancer in Sussex are lonely as a result of their illness.
Macmillan Cancer Support says many patients end up becoming housebound and unable to feed themselves properly because of their isolation.
The charity estimates there are around 8,500 cancer patients in the county who feel they are on their own and struggling to cope.
A survey carried out for the charity also found that as well as feeling lonely, patients sometimes also drink more, stay at home for several days at a time, skip meals and struggle to sleep.
Ann Brady from Hastings was diagnosed with breast cancer in 2011. She said: “When I was back at home I did start to feel quite isolated and could have done with some more support.
“I really needed to speak to someone who’d been through what I’d been through – for example, was the pain I was feeling normal, or not normal?
“I think what I most wanted was reassurance. I wanted to ask someone like me: ‘did you feel like that too?’”
Macmillan Cancer Support director for England, Fran Woodard, 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.”
Macmillan provides a range of services to provide help, including a support line and an online community, but needs more donations and support to keep up with demand.
It also runs volunteer schemes such as Brighton Buddies, where someone visits a patient once a week for three months to lend a listening ear and help with small practical tasks.
For more details 0808 808 00 00 or visit www.macmillan.org.uk.