The idea that antibiotics may cause breast cancer was first given a lot of attention upon publication of a 2004 study that reported a connection between the two. The study was roundly criticized by other researchers at the time for its lack of control for other patient characteristics known to contribute to breast cancer risk. Studies published since then have found either a small positive association between breast cancer and antibiotic use or no significant association. Some very large population studies have been undertaken to try to settle this question. However, the prescription and cancer databases that facilitate such large studies are typically not able to provide data concerning confounding factors for all of the subjects. Another problem is the lack of a completely convincing mechanism of action, i.e., a compelling explanation of the way in which antibiotics might contribute to breast cancer development.
Some antibiotics have been associated with increased breast cancer risk
Available evidence suggestds that some antibiotics might promote breast cancer in some women. Future studies might produce more specific and definitive results concerning individual medications and breast cancer type. The following antibiotics and antifungals have been found to be associated with increased breast cancer rates:
- Cipro (ciprofloxacin)
- Flagyl (metronidazole)
- Garamycin (gentamicin)
- Grisovin (griseofulvin)
- Tetracyclines such as Sumycin, Terramycin, Tetracyn, and Panmycin.
We suggest making every effort to avoid long-term antibiotic use for less serious conditions such as acne and rosacea. Generally speaking, topical antibiotics (if effective for a given condition) are preferable to oral antibiotics.
Possible reasons for link between antibiotics and breast cancer
Several explanations have been proposed for a possible connection between antibiotic use and breast cancer. According to one theory, antibiotics interfere with the ability of intestinal microflora to metabolize phytochemicals from edible plants into compounds that may protect against cancer. Another theory suggests that antibiotic use may be associated with breast cancer risk by influencing immune and inflammatory factors. A study of mice that spontaneously develop mammary tumors that overexpress HER-2/neu (HER2+) found that mice administered either Garamycin or a combination of Cipro and Flagyl developed more tumors than similar untreated mice. The mammary glands of the mice who were treated were found to have more numerous and more developed terminal ductal lobular units than control mice. This suggests a more direct possible mechanism of action in which the breasts themselves are altered by antibiotic exposure.
On the other hand, it is possible that the use of antibiotics simply reflects a health deficit such as chronic inflammation, chronic infection, or an overall weakened immune system that is responsible for increased breast cancer risk.
Antibiotics in food
Antibiotics are routinely administered to livestock (including poultry) and farmed fish to help reduce infections in the crowded conditions in which they are raised. Milk typically contains measurable quantities of antibiotics. Based on other evidence concerning these foods and the risk of breast cancer, beef, pork, lamb, and milk should be avoided, and organic rather than non-organic chicken (and other poultry) should be purchased, if possible. Wild salmon and other fatty fish are preferable to farm raised fish because they normally have higher omega-3 content, in addition to not having been raised using antibiotics and pesticides.
Bottom line
Based on the available evidence, we conclude that the association between overall antibiotic use and breast cancer is not strong and causality has not been shown. Women should continue to follow the prevailing medical advice to limit antibiotic use to when it is strictly medically necessary and to complete any course of antibiotics once it has been started. Serious infections can be deadly or cause permanent injury and should be treated appropriately.