Interest in using very high doses of vitamin C as a cancer treatment began as long ago as the 1970s 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 in pill form found no such benefits for people with cancer. Despite the lack of evidence, some 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 alone can cure cancer, but researchers are studying whether it might boost the effectiveness of other cancer treatments, such as chemotherapy and radiation therapy, or reduce treatment side effects.
There are still no large, controlled clinical trials that have shown a substantial effect of vitamin C on cancer, but some preliminary studies do suggest there may be a benefit to combining standard treatments with high-dose IV vitamin C. Until clinical trials are completed, it's premature to determine what role vitamin C may play in the treatment of cancer.