Large doses of vitamin C (ascorbic acid) have been proposed as treatments or cures for various types of cancer, including breast cancer. Vitamin C is a micronutrient vital to human health. Relatively high intakes of dietary vitamin C (i.e., from consuming food) have been found to be associated with lower risk of childhood leukemia, as well as oral, skin, gastric, pancreatic, colon, endometrial, and prostate cancer. Moderate vitamin C supplementation has also been found to be associated with lower risk of some cancers, among them bladder cancer in men and colon cancer in women. Relatively high dietary intake of vitamin C has been shown to be associated with lower breast density in young women.
The case for vitamin C supplementation is based, in part, on the observation that 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. Laboratory studies have found that vitamin C at various concentrations inhibits human breast cancer cell growth. However, we are not aware of any academic studies that demonstrate a survival benefit for women being treated with high doses of vitamin C.
Recent evidence suggests that vitamin C should not be taken during treatment with tamoxifen since it has been shown to protect breast cancer cells against a type of cell death caused by tamoxifen.
Based on the available evidence, while it appears that dietary vitamin C might protect against breast cancer, supplementation with relatively high doses of vitamin C may not be beneficial for breast cancer patients or survivors. In fact, the evidence hints that such supplementation could actually promote cancer.

Dietary sources of vitamin C

The following foods are good dietary sources of vitamin C while also having been shown to protect against breast cancer:
Bell peppers
Brussels sprouts
Collard greens
Hot peppers
Lettuce, romaine
Mustard greens
Vitamin C appears to act synergistically with other nutrients in the diet to oppose cancer. On the other hand, high doses of vitamin C may extinguish the anti-cancer effects of some other micronutrients known to be protective against breast cancer.

Bottom line

Most breast cancer patients, survivors and those at high risk are in a good position to obtain their vitamin C through a tailored breast cancer diet. Supplementation with vitamin C is not necessary and treatment with high doses of vitamin C could be counterproductive. Vitamin C supplements should not be taken during chemotherapy or tamoxifen treatment.
Below are links to recent studies on this topic. For a more complete list of studies, please click on vitamin C.