What are free radicals?
Free radicals are unstable molecules that can damage whatever they bump into, including DNA, proteins and cell walls, but in small amounts your body also uses them as important signals.
Think of sparks from a campfire. A few sparks are harmless and even useful; they tell you the fire is burning. Too many, landing on the wrong things, start damaging the tent.
Why it matters for aging
Free radicals are produced naturally, mostly inside mitochondria, the cell's power plants, as a by-product of making energy. Sunlight, smoking, pollution and inflammation produce more. When they outnumber the body's defenses, the result is called oxidative stress.
For decades, scientists thought free radicals were the main cause of aging. Newer evidence paints a more nuanced picture: in mitochondrial DNA, for example, most age-related mutations look like copying errors, not free-radical damage. And short bursts of free radicals after exercise help trigger the body's own repair and adaptation.
A common myth
Free radicals cause aging, so antioxidant pills that mop them up should slow it.
Free-radical damage plays a role, but the "free radical theory of aging" turned out to be too simple. Your body uses free radicals as signals, for example to adapt to exercise, and large trials of antioxidant pills didn't extend life; some were linked to harm.
Go deeper
Deep dives
- The best supplements for longevity, graded by evidence
- The first part of you to break isn't in your nucleus
Related terms
Updated Nov 9, 2026 · All terms