In the study, patients who took the pill, along with two or more cups of coffee daily, reported less than half the rate of cancer recurrence, compared with their non-coffee drinking, Tamoxifen-taking counterparts.
The team followed over 600 breast cancer patients from southern Sweden for an average of five years. Approximately 300 took Tamoxifen. The drug, a common hormone therapy after breast cancer surgery, reduces the risk of new tumours by blocking oestrogen receptors.
How coffee interacts with the treatment, however, isn't immediately known.
"One theory we are working with is that coffee 'activates' Tamoxifen and makes it more efficient", said Maria Simonsson, doctoral student in Oncology at Lund University.
The Lund University researchers have previously linked coffee consumption to a decreased risk of developing certain types of breast cancer.
Caffeine has also been shown to hamper the growth of cancer cells. The latest observational study involving coffee's role in cancer prevention and treatment underlines the need for more research, the team noted.