Oral doxycycline is one of the most frequently prescribed medications by optometrists. Doxycycline, a tetracycline derivative and broad-spectrum antibiotic, is commonly used to treat various inflammatory ocular conditions, such as keratoconjunctivitis sicca, meibomitis, rosacea keratitis, corneal ulcers, chlamydial keratoconjunctivitis and recurrent corneal erosion.1-4
Doxycycline also inhibits production of matrix metalloproteinase (MMP), an enzyme that causes collagen destruction.1,2
However, in recent years, there has been discussion about a potential relationship between oral doxycycline and an increased risk for breast cancer. But, is there enough evidence to suggest a positive link?
The Evidence in Favor
Eight years ago, an original study published in the British Journal of Cancer suggested a link between antibacterial medications, such as oral tetracycline and its derivatives, and breast cancer.5 Specifically, this study found that a patients lignan status is markedly reduced by repeated use of antibacterial medications.5 Lignans, a group of antioxidant chemical compounds found in plants, are believed to protect against breast cancer.6
Ultimately, the results were inconclusive, and the authors suggested that other studies are necessary to test and confirm their hypothesis.
In another study, researchers evaluated the association between breast cancer and antibiotic use in 2.1 million women. At a nine-year follow up, the researchers found that 18,521 women had developed breast cancer.7 The study concluded that use of antibiotics, such as tetracyclines and macrolides, is associated with a slightly increased risk for breast cancer.7
Doxycycline is often used to treat meibomitis, as seen in this patient.
The Evidence Against
However, a literature search uncovered many other studies that show no association between antibiotics and breast cancer incidence.
In fact, several studies conclude that tetracyclines may actually help prevent breast cancer and suppress cancerous cell growth, invasion and metastasis.8
Besides inhibiting MMP activity, oral tetracyclines reduce levels of MMP expression and exhibit selective cytotoxicity toward some of the tumor cell lines.9,10
Specific to breast cancer, research shows that chemically modified tetracycline inhibits secretion of vascular endothelial growth factor (VEGF); this, in turn, restricts tumor invasiveness and metastasis.11
Inhibition of VEGF secretion also halts blood vessel growth, a key factor in cancer progression. New blood vessels directly feed malignant tumors. A study on chicken chorioallantoic membranes during fetal development shows that doxycycline can prevent new blood vessel formation. This suggests that the drug could be used as a potential breast cancer therapy.2
Another study in 2005 showed that doxycycline made anti-tumor medications, such as cyclophosphamide, more effective.12
Studies also show that tetracyclines effectively treat prostate cancer by delaying tumor growth, reducing metastasis and decreasing the incidence of tumors.13 Finally, additional studies have demonstrated that bone metastasis decreased in breast cancer patients who took doxycycline.14,15
Based on the existing research, there is little conclusive evidence to suggest a positive relationship between doxycycline use and an increased risk for breast cancer. Quite the contrary, most documented research concludes that doxycycline may be an effective treatment option for breast cancer.
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