"A healthy diet is the most effective way to get the antioxidants your body needs. Fruits, vegetables, grains, eggs and nuts are all useful sources of antioxidants. Despite the marketing hype, antioxidants found in so-called superfoods are no more effective than those in regular fruit and veg, so you’re better off saving your money.
But it’s a different story when it comes to antioxidant supplements. Research has found antioxidant supplements may cause more harm than good. A 2012 meta analysis of over 70 trials found antioxidant supplements are ineffective or even detrimental to health. The reasons are unclear, but the added nutritional benefits from consuming antioxidants in a healthy diet is likely to contribute to this. Also, the high concentrations of antioxidants associated with supplement use can lead to problems."
Jacqui Adcock, Research Fellow in Analytical Chemistry, Deakin University, explains how antioxidants work and why too much of a good thing may be harmful:
Personally, I've heard conflicting opinions regarding whether taking antioxidants, like green tea/EGCG during chemotherapy is wise. One school of thought is that antioxidants help the cancer cells repair the DNA damage caused by treatment - and hence protecting the cancer!