Cancer

Results of many population-based studies suggest that eating foods rich in vitamin C may be associated with lower rates of cancer, including skin cancer, cervical dysplasia (changes to the cervix which may be cancerous or precancerous, picked up by pap smear), and, possibly, breast cancer. But these foods also contain many beneficial nutrients and antioxidants, not only vitamin C, so it's impossible to say for certain that vitamin C protects against cancer. Taking vitamin C supplements, on the other hand, has not been shown to have any helpful effect.
In addition, there is no evidence that taking large doses of vitamin C once diagnosed with cancer will help your treatment. In fact, some doctors are concerned that large doses of antioxidants from supplements could interfere with chemotherapy medications. More research is needed. If you are undergoing chemotherapy, talk to your doctor before taking vitamin C or any supplement.