Investigations › Supplements
Investigation No. 15 · Supplements
“Antioxidant supplements prevent cancer and aging”
Seventy-eight randomized trials in 296,707 people, and the authors’ conclusion is that these supplements may increase mortality.
At a glance
- Verdict
- False, and risky
- Certainty in the evidence
- Moderate, starting at High and moved by 1 domain
- Evidence base
- Cochrane review: 78 randomized trials · 78 studies read · 296,707 people
- Last reviewed
- 11 Sept 2026
- Next review due
- Sept 2027
How people actually say it
- “I take them for the free radicals.”the mechanism, quoted as though it were the outcome
- “It can’t hurt to take extra.”the assumption this entry exists to correct
Why this verdict and not the next one
This moved from mixed to risky on reading the Cochrane review. Mixed implied the studies disagree about whether there is a benefit. What the largest pooling found was a signal pointing the other way, which is a different verdict.
Where the claim came from
From a real and elegant mechanism. Oxidative damage accumulates, antioxidants neutralise it in a test tube, and the inference that swallowing more would slow ageing is the kind of reasoning that feels like it must be right. It is also a mechanism, and a mechanism is not an outcome.
What the studies did
Bjelakovic and colleagues searched six databases to 2011 and pooled 78 randomized primary and secondary prevention trials covering 296,707 participants, of which 56 trials and 244,056 participants were at low risk of bias. Mean age 63. They ran trial sequential analysis to control for random error and meta-regression to look for the sources of heterogeneity. Their conclusion is that antioxidant supplements may increase mortality.
Why these papers and not others
The Cochrane review was chosen over any single trial because it pools 78 of them, and over newer narrative reviews because it states its search, its risk-of-bias assessment and its uncertainty. Its 2011 search cutoff is named as the reason to re-read it.
The full search and appraisal protocol, including what gets excluded and why, is set out in the method.
The evidence, study by study
| Study | Design | n | What it found | What it cannot say |
|---|---|---|---|---|
| Bjelakovic 2012 | Cochrane systematic review of randomized trials | 296,707 | Beta-carotene, vitamin A, vitamin C, vitamin E and selenium against placebo or no intervention. The authors conclude that antioxidant supplements may increase mortality. | Which specific nutrient at which dose drives the signal, since the review pools a category |
| Same review, subset | 26 trials in healthy participants | 215,900 | The healthy-participant subset is large enough that this is not a finding about sick people only | Anything about food-form antioxidants, which were not the intervention |
How the certainty grade was reached
Certainty starts at a level set by the study design, then moves down for every domain that fails a written threshold and up for every domain that clears one. The arithmetic is shown rather than asserted.
What the evidence cannot say
Separate the nutrients cleanly. Pooling beta-carotene, vitamin A, vitamin C, vitamin E and selenium as one category is what makes the review large and what makes it blunt. It also says nothing about antioxidants in food, which were never the intervention.
One person, worked through
Somebody in their sixties adds a high-dose antioxidant formula because a mechanism made sense to them. The evidence base under that decision is not absent, which would be one thing. It is large, and it points the wrong way.
Whether this applies to you
A verdict is a reading of a population. Whether it describes you depends on the things below, and any one of them can change what the right move is for you.
- Food-form antioxidants are a different question and were not what these trials tested
- A diagnosed deficiency is a different question again since correcting a deficiency is not supplementation for prevention
- Beta-carotene in smokers is the specific combination with the clearest harm signal in the older trials
- The review closed its search in 2011 which is the strongest argument for re-reading it
What would change this entry
An updated review with the trials published since 2011, and analyses that separate the nutrients rather than pooling them as a category.
Verdict changed · Was ‘evidence is mixed’ until 11 Sep 2026
Read this before you act on it
This is an appraisal of published evidence, not medical or nutrition advice, and it is not a substitute for care from a clinician who knows your history, your medications and your labs. Nothing here is a diagnosis, a prescription or a treatment plan, and reading it does not create a clinician and patient relationship.
Do not start, stop or change a medication, a supplement or a diet on the strength of this page. If you are pregnant, breastfeeding, managing kidney or liver disease, diabetes, an eating disorder or any chronic condition, the general reading above may not describe you at all. Talk to your physician or a registered dietitian first.
The verdict above was true to the evidence on 11 Sept 2026. Evidence moves. Check the review date.
References
- Bjelakovic G, Nikolova D, Gluud LL, Simonetti RG, Gluud C. Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database of Systematic Reviews. 2012 Mar 14;2012(3):CD007176. doi:10.1002/14651858.CD007176.pub2. PMID 22419320.
