Relatively high intakes of dietary vitamin C (i.e., from consuming food) have been linked to lower risks of various types of cancer, including breast cancer, although the evidence is not consistent. Laboratory studies have found that vitamin C at various concentrations inhibits breast cancer cell growth.
Breast cancer patients tend to have low blood levels of antioxidants compared with healthy women, indicating that the patients have exhausted the antioxidant defenses of the body and are vulnerable to oxidative damage that could further promote breast cancer. Based on these observations, vitamin C (in some cases, very large doses) has been proposed as part of the treatment for breast cancer.
Large studies examining typical levels of vitamin C supplementation (included those found in multivitamins) have produced mixed results. Supplemental vitamin C intake has been found both to be weakly associated with improved survival and to have no impact on prognosis. No study has demonstrated a treatment benefit for high doses of vitamin C and researchers have abandoned the study of vitamin C as a potential cancer cure. Now a new study has reported that vitamin C can protect breast cancer cells from tamoxifen treatment, thereby potentially reducing its effectiveness.

Latest research finds vitamin C protects breast cancer from tamoxifen

The study referenced at the beginning of this news story was designed to investigate the influence of vitamin C on tamoxifen-mediated cell death. Tamoxifen inhibits estrogen receptor positive (ER+) cell growth and proliferation by interfering with the cells' ability to use estrogen. However, tamoxifen also kills breast cancer cells directly by inducing apoptosis (programmed cell death).
To conduct the study, the authors loaded breast cancer cells with vitamin C before applying tamoxifen treatment. Pre-treatment with vitamin C caused a dose-dependent reduction in cell death by protecting the cancer cells against several of the cytotoxic effects of tamoxifen. For example, a reduction in the release of pro-apoptotic factors from the mitochondria normally caused by tamoxifen was observed. The authors conclude that vitamin C supplementation during cancer treatment may detrimentally affect therapeutic response.
Please see our articles on what to eat during tamoxifen treatment and vitamin C for more information.