The relationship between alcohol and breast cancer has been a subject of significant research. It has long been considered a risk factor for breast cancer in women.^[1][2] The International Agency for Research on Cancer has declared that there is sufficient scientific evidence to classify alcoholic beverages a Group 1 carcinogen that causes breast cancer in women.^[3] Group 1 carcinogens are the substances with the clearest scientific evidence that they cause cancer, such as smoking tobacco.

A woman drinking an average of two units of alcohol per day has 8% higher of developing breast cancer than a woman who drinks an average of one unit of alcohol per day.^[4] A study of more than 1,280,000 middle-aged British women concluded that for every additional drink regularly consumed per day, the incidence of breast cancer increases by 1.1%.^[5] Approximately 6% (between 3.2% and 8.8%) of breast cancers reported in the UK each year could be prevented if drinking was reduced to a very low level (i.e. less than 1 unit/week).^[4]

Among women, breast cancer comprises 60% of alcohol-attributable cancers.^[6]

A study of 17,647 nurses found that high drinking levels more than doubled risk of breast cancer with 2% increase risk for each additional drink per week consumed. Binge drinking of 4–5 drinks increases the risk by 55%.^[7]

The mechanisms of increased breast cancer risk by alcohol may be:

• Increased estrogen and androgen levels^[8]
• Enhanced mammary gland susceptibility to carcinogenesis^[8]
• Increased mammary DNA damage^[8]
• Greater metastatic potential of breast cancer cells^[8]

Their magnitude likely depends on the amount of alcohol consumed.^[8]

Susceptibility to the breast cancer risk of alcohol may also be increased by other dietary factors, (e.g. folate deficiency), lifestyle habits (including use of hormone replacement therapy), or biological characteristics (e.g. as hormone receptor expression in tumor cells).^[8]

A study on mice suggests that, when breast cancer is established, drinking as little as two alcoholic drinks a day increases the growth rate of tumors. Alcohol causes cancer cells' blood vessels to grow which in turn fuels the growth of the tumor, a process known as angiogenesis.^[9]

Studies suggest that drinking alcohol during pregnancy may affect the likelihood of breast cancer in daughters. "For women who are pregnant, ingestion of alcohol, even in moderation, may lead to elevated circulating oestradiol levels, either through a reduction of melatonin or some other mechanism. This may then affect the developing mammary tissue such that the lifetime risk of breast cancer is raised in their daughters."^[10]

Studies are ambiguous, with some stating that the type of drink is not a factor,^[11] while one study suggests that women who frequently drink red wine may have an increased risk of developing breast cancer.^[12]

Moderate alcohol consumption has also been found to increase the breast cancer risk.^[13]

One of the largest studies of its kind has found that alcohol is a substantial risk factor for development of the most common type of breast cancer - the 70% of tumors that are classified as positive for both the estrogen and progesterone receptors (ER+/PR+). Researchers report that even moderate alcohol consumption, defined as one or two drinks per day, increased risk of developing this kind of cancer, and the more a woman drank, the higher her risk. Compared to women who did not drink at all, women who had three or more glasses of alcohol daily had as much as a 51% increased risk of ER+/PR+ breast cancer.^[14]

In contrast, research by the Danish National Institute for Public Health, comprising 13,074 women aged 20 to 91 years, found that moderate drinking had virtually no effect on breast cancer risk.^[15]

Moderate to heavy consumption of alcoholic beverages (at least three to four drinks per week) is associated with a 1.3-fold increased risk of the recurrence of breast cancer.^[16][17]

In men, breast cancer is rare, with an incidence of fewer than one case per 100,000 men.^[18] Population studies have returned mixed results about excessive consumption of alcohol as a risk factor. One study suggests that alcohol consumption may increase risk at a rate of 16% per 10g daily alcohol consumption.^[19] Others have shown no effect at all, though these studies had small populations of alcoholics.^[20]

• UK: Committee on Carcinogenicity of Chemicals in Food, Consumer Products Consumption of alcoholic beverages and risk of breast cancer
• UK: Committee on Carcinogenicity of Chemicals in Food, Consumer Products Evidence for association between consumption of alcoholic beverages and breast cancer