The appealing theory — and the disappointing results
Antioxidants neutralize 'free radicals,' so the logic seemed sound: take more antioxidants, prevent the damage behind aging and disease. But when high-dose antioxidant supplements were put to the test in large trials, the theory largely fell apart. NCCIH summarizes that high-dose antioxidant supplements have not been shown to prevent chronic diseases and in some cases caused harm [1].
What the big trials found
- Beta-carotene and lung cancer. High-dose beta-carotene 'may increase the risk of lung cancer,' particularly in people who smoke or were exposed to asbestos [1]. This was one of the clearest 'antioxidant backfire' findings.
- Cardiovascular outcomes. NCCIH notes the evidence does not support antioxidant supplements preventing cardiovascular disease, and beta-carotene was linked to higher cardiovascular death [1].
- Vitamin E and prostate cancer. Large trials raised concerns rather than showing benefit.
- Cataracts. A review of high-quality studies found no effect on cataract occurrence or progression [1].
In 2022 the U.S. Preventive Services Task Force recommended against using beta-carotene or vitamin E supplements for lowering cancer risk [1].
Why food is different from a megadose
Antioxidants in fruits, vegetables, nuts, and whole grains come in modest amounts, alongside fiber and many other compounds. Isolating one antioxidant and taking it at many times the dietary dose is a different intervention — and the body's redox balance is something high doses can disrupt rather than help. This is a textbook case of the dose-response relationship: more is not better.
Practical guidance
- For general health, get antioxidants from a varied diet, not high-dose pills (see [food-first](/learn/do-you-need-supplements-food-first)) [2].
- Be especially cautious with high-dose beta-carotene if you smoke, and with high-dose [vitamin E](/learn/vitamin-e-intake-and-bleeding).
- Specific clinical uses (such as the AREDS formula for certain eye conditions) are different and physician-directed — they don't justify routine high-dose antioxidant use for everyone.