Vitamin C (Ascorbic Acid)
In plain terms
The antioxidant vitamin used in serums to brighten, fade dark spots, and support collagen. It is benign; the only catch is that the pure, most-active form is acidic enough to sting sensitive skin.
What the evidence shows
Vitamin C is an antioxidant your body needs but cannot make, which is part of why it is so well tolerated on skin: it is working with biology, not against it. Topically it neutralizes the free radicals that UV and pollution generate, acts as a cofactor for building collagen, and interferes with the enzyme that drives dark spots, so it earns its place in brightening and anti-aging serums. On safety, dermatology is direct that topical vitamin C and its derivatives are safe and not linked to adverse effects. The one real catch is chemistry, not toxicity. The pure, most-active form, L-ascorbic acid, only penetrates skin at a low, acidic pH, and that acidity can sting or redden sensitive skin, especially above about 20%, where it stops absorbing better and only irritates more. The fix is simple: a lower strength, or a gentler derivative like sodium ascorbyl phosphate, which works at a friendlier pH. The other quirk people notice, a serum turning yellow then brown, is oxidation: it means the vitamin C has lost potency, a freshness problem rather than a hazard. There is no hormone, cancer, or pregnancy question attached to it.
On the label
Listed as ascorbic acid or L-ascorbic acid, or as a gentler derivative like sodium ascorbyl phosphate or ascorbyl glucoside. If the pure form stings, a derivative or lower strength solves it; a serum gone orange-brown has oxidized and lost potency, which is a freshness issue, not a safety one.