Bowel cancer sufferers are nearly three times more likely to have clinical depression if they do not have emotional support, a study has found.
Those without someone to confide in or ask for advice were also twice as likely to have anxiety and low levels of well-being, the study found.
Researchers from Southampton University and Macmillan Cancer Support followed more than 1,000 people with the disease from before surgery until five years afterwards.
They found a sharp reduction in social interaction, affection and practical support after operations.
Macmillan chief executive Lynda Thomas said it showed the "heartbreaking reality for thousands of people with cancer" with no-one to talk to or "cook them a meal when they're wiped out from chemotherapy".
She added: "It's hard enough going through cancer when you have support, but without it, you must feel completely lost."
Findings also included a trebling of patients who said they had "little or no affection" two years after diagnosis as well as a doubling of those who lacked practical help.
The odds of suffering from anxiety or depression also doubled for those living alone.
Ron Peck, 55, a teacher from Sussex, was diagnosed with bowel cancer in 2011 and felt "isolated and abandoned" during treatment.
He said: "I'd been warned that the support you have during your treatment can suddenly fall away. I thought I was prepared for it but I ended up feeling emotionally isolated and abandoned.
"There was a void where my care used to be and there was no-one to talk to about my ongoing side-effects.
"Many of your friends who initially rallied around move on too. They don't appreciate the continued impact that cancer has on you."
The Colorectal Wellbeing (CREW) study was led by professor Claire Foster with findings released on Monday at the international psycho oncology society congress.
Professor Foster said: "People can feel isolated following their treatment and those with limited social support are at greater risk of this.
"More needs to be done to identify and help people who are struggling in the months and years following cancer treatment."
Macmillan offers information and advice to people affected by cancer on its website www.macmillan.org.uk/carers and through its support line on 0808 808 00 00.