Coffee could make breast cancer drug tamoxifen more effective


"A cancer-killing cocktail of the hormone drug tamoxifen and two coffees every day was found to reduce the risk of [breast cancer] tumours returning," the Mail Online reports. The same study also found evidence that caffeine slowed the cancer's growth…

"A cancer-killing cocktail of the hormone drug tamoxifen and two coffees every day was found to reduce the risk of [breast cancer] tumours returning," the Mail Online reports. The same study also found evidence that caffeine slowed the cancer's growth.

The study looked at coffee consumption among 1,090 women with breast cancer, about half of whom were treated with tamoxifen.

Tamoxifen is a hormonal treatment used to treat cases of breast cancer known to be associated with the hormone oestrogen (known as oestrogen-dependent breast cancer).

The study found that women who reported drinking two to five cups of coffee a day had smaller primary tumours and a lower proportion of oestrogen-dependent tumours than those who drank one cup of coffee or less.

Women with oestrogen-dependent breast cancer who were treated with tamoxifen, and who drank at least two to five cups of coffee daily, had half the risk of the cancer recurring as those who drank less.

Researchers also carried out a laboratory study on the effect of two substances found in coffee – caffeine and caffeic acid (a compound found in coffee) – on breast cancer cells. They found that the substances suppressed the growth of breast cancer cells.

Although the results of this study are interesting, it cannot prove that coffee has an effect on breast cancer, as other factors, called confounders, could have influenced the results. 

There’s no harm in women being treated with tamoxifen for breast cancer drinking coffee in moderation. However, drinking excessive amounts can cause irritability, insomnia and indigestion.

Where did the story come from?

The study was carried out by researchers from Lund University and Skane University Hospital in Sweden, and the University of Bristol in the UK. It was funded by various grants from Swedish organisations, including the Swedish Cancer Society and the Swedish Research Council.

The study was published in the peer-reviewed journal Clinical Cancer Research.

The Mail Online’s reporting of the results implies it was proven that coffee prevents the return of oestrogen-dependent breast cancer or enhancing the action of tamoxifen. This is not the case, though the initial results are encouraging.

The Mail Online also did not include any comments on the study from independent experts. As a result, there is a risk that millions of women taking tamoxifen will start worrying about how much coffee they should drink.

There are no official UK guidelines on caffeine consumption, but regularly drinking more than 400 milligrams (mg) of caffeine a day (around four cups of brewed coffee or two "energy drinks") could cause side effects.

What kind of research was this?

This was a cohort study of 1,090 women with primary invasive breast cancer, living in Sweden. It is a follow-up study to one published in 2013 by the same researchers, which used a smaller number of women from the same cohort. The researchers say their previous study found an association between moderate to high coffee consumption and improved survival rates among women with breast cancer who were treated with tamoxifen. The researchers say the aim of the study is to investigate the association between coffee consumption, cancer characteristics and survival rates in a larger cohort of women with breast cancer.

Some breast cancer tumours rely on oestrogen to grow. These are called oestrogen-receptor (ER) positive cancers (the convention in classifying these types of cancer is to use the American spelling of oestrogen, which is estrogen; hence the ER).

Tamoxifen is the main hormonal therapy drug given for these types of breast cancer, as it blocks oestrogen from reaching the cancer cells. This reduces or stops the cells from growing.

The researchers also performed studies in the laboratory using human breast cancer cells to look at possible mechanisms by which two substances in coffee – caffeine and caffeic acid – may affect breast cancer growth.

What did the research involve?

For the cohort study, 1,090 women aged 24 to 99 who had been diagnosed with primary invasive breast cancer between 2002 and 2012 were recruited. Before undergoing surgery, the women’s body measurements and breast volume were taken and they were given an extensive questionnaire on their reproductive history, medication use and lifestyle factors, including smoking, alcohol and coffee consumption.

Coffee consumption was categorised into low (one cup or less a day), moderate (two to four cups a day) or high (five or more cups a day).

The researchers obtained information from pathology reports and medical records about tumour size and grade, whether it had spread to any lymph nodes, and if the tumour was hormone-receptor positive.

The women were followed until either their first breast cancer recurrence, their last disease-free follow-up or their death, whichever came first, before January 2013. Follow-up information on whether the breast cancer came back or whether the women had died was obtained from various official records. The results were analysed using standard statistical methods and adjusted for other factors, such as tumour size.

In their laboratory study, human breast cancer cells were exposed for 48 hours to caffeine or caffeic acid, with or without tamoxifen. The researchers used breast cancer cells that were ER positive, ER negative or cells that were resistant to tamoxifen. A minimum of three independent repeats were performed for each experiment.

What were the basic results?

The main results of the cohort study were:

  • women who reported a moderate to high coffee intake had smaller invasive primary tumours compared to those with low coffee consumption
  • moderate to high coffee intake was also associated with a lower proportion of ER positive tumours compared to patients with low consumption
  • moderate to high coffee consumption was associated with a 49% lower risk for breast cancer recurrence in women with ER positive tumours being treated with tamoxifen (adjusted hazard ratio 0.51; 95% confidence interval 0.26-0.97)

In the laboratory, caffeine and caffeic acid suppressed the growth of both ER positive and ER negative cancer cells. Caffeine and caffeic acid also had other effects on breast cancer cells, which led to slower cell growth and enhanced cell death.

How did the researchers interpret the results

The researchers say their findings demonstrate the various anticancer properties of caffeine and caffeic acid against both ER positive and ER negative breast cancers. In particular, they suggest that coffee may sensitise tumour cells to tamoxifen and therefore reduce breast cancer growth. It is possible, they say, that the substances in coffee switch off signalling pathways that cancer cells need to grow.


This study is interesting, but has several limitations. Its first finding was that women who report higher coffee consumption have smaller breast tumours, and also that their cancers are less likely to be ER positive. However, it seems that the women only reported their coffee consumption once, after diagnosis, and it is unclear from the write-up whether the questionnaire referred to their past or present coffee drinking habits. The women may also have under- or overestimated their coffee consumption, especially if they were asked to recall coffee consumption over a long period. The accuracy of coffee consumption is further hampered, as the study did not provide a standard definition for the size of a "cup" of coffee.

The second finding was that among women with ER positive cancer being treated with tamoxifen, higher coffee consumption was associated with better results. This sounds promising, especially when taken with the results of the laboratory study, but it is always possible that confounders might have affected the results.

Tamoxifen is normally only used in women with ER positive cancer who have not yet gone through the menopause. Therefore, it is unclear whether a similar effect would be seen in post-menopausal women who require a different type of hormonal treatments, such as aromatase inhibitors.

Further research is required on the possible association between coffee consumption and breast cancer risk, as it could lead to new treatments.

However, it should be noted that over-consuming coffee can have negative side effects. Regularly drinking more than five cups of coffee a day can cause insomnia, irritability, an upset stomach and palpitations.

Article Metadata Date Published: Tue, 15 Aug 2017
Author: Zana Technologies GmbH
NHS Choices