Many are missing appointments, not taking their medication properly, are unable to pick up prescriptions or even refusing treatment, said Macmillan Cancer Support.
The charity said such patients were almost three times more likely to encounter such problems as those who are not lonely.
Its poll of more than 1,000 people who had ever been diagnosed with the disease found 22 per cent have been lonely after diagnosis. Almost a third (31 per cent) of those who stated they had been lonely said they faced at least one issue with their treatment plan, against 11% who had not felt alone.
Jacqui Graves, head of health and social care at Macmillan Cancer Support, said: "We know lonely patients who have attended appointments only because friends or family persuaded them."
Ciaran Devane, the charity's chief executive, said: "We already know loneliness may be as harmful as smoking but this research shows for the first time it is particularly toxic to cancer patients.
"That is why we are calling on health professionals to identify lonely cancer patients and make them aware of the support available so they do not have to go through their cancer alone."