Vitamin C is quite safe when supplemented in food form. However, high doses of vitamin C can cause side effects, the most common of which are digestive symptoms, and can have serious consequences such as iron deposits, kidney stones, etc.

Some researchers have found that high-dose vitamin C as an immunotherapy boost slows tumor growth. However, they also note that the major drawback of using high-dose vitamin C to treat cancer in humans is the potential for side effects.

Currently, there are many studies on intravenous injection of high doses of vitamin C that have the ability to attack cancer cells without harming health. However, these studies have mostly been done in test tubes or in mice, but very few studies have been done in humans.