Interest in using very high doses of vitamin C as a cancer treatment began when it was discovered that some properties of the vitamin may make it toxic to cancer cells. Initial studies in humans had promising results, but these studies were later found to be flawed.
Subsequent well-designed, randomized, controlled trials of vitamin C and cancer found no such treatment benefit. Despite the lack of evidence, alternative medicine practitioners continue to recommend high doses of vitamin C for cancer treatment.
More recently, vitamin C given through a vein (intravenously) has been found to have different effects than vitamin C taken in pill form. This has prompted renewed interest in the use of vitamin C as a cancer treatment.
There's still no evidence that vitamin C can cure cancer, but researchers are studying whether it might boost the effectiveness of other cancer treatments, such as chemotherapy and radiation therapy. Until clinical trials are completed, it's premature to determine what role, if any, intravenous vitamin C may play in the treatment of cancer.