Depression increases the risk of dying for women with breast cancer.
Patients diagnosed with depression had a 45 per cent higher risk of death from all causes than those who were not depressed, new research has found.
The trend emerged after scientists followed the progress of 77,173 women in southeast England with breast cancer for up to 10 years.
It remained after taking into account other factors that may have affected survival including age at cancer diagnosis, stage of cancer and socio-economic status.
Lead researcher Dr Elizabeth Davies, from King's College London, said low mood and depression were understandable reactions to a breast cancer diagnosis.
"Clinicians generally know to look out for this, but these findings emphasise the need to ask patients with cancer about their mood and for women to know it's okay to ask for help.
"Greater social support or psychological interventions for women with breast cancer could help to reduce the negative effects amongst those most at risk of depression."
Depression-linked behaviours such as adopting a less healthy lifestyle, chronic stress or non-compliance with treatment, could help explain the link, said the researchers whose results are published in the journal Psycho-Oncology.
Overall, 55 per cent of women with a record of depression were alive five years after their breast cancer diagnosis compared with 75 per cent with no depression history.
The researchers pointed out that less severe depression may not be recorded on medical notes such as those used in the study.
A mention of depression was seen in 1.2 per cent of breast cancer patients' medical records.
Other studies that questioned women with breast cancer directly found evidence of depression in up to a quarter of patients.
