Identity
Caffeine (INCI: Caffeine; chemically 1,3,7-trimethylxanthine) is the same small, well-characterised molecule found in coffee and tea, used in skincare as a purified active. It is worth drawing one line early: caffeine the isolate is not "coffee extract" — Coffea arabica seed extract, guarana, or green-tea extract are whole-plant materials with many constituents, and they are graded on their own, not as caffeine. This entry is about the isolated molecule.
Caffeine is one of the most widely used cosmetic actives, appearing across eye creams, antioxidant serums, and "firming" body formulas — which is exactly why its claims run such a wide range, from the well-evidenced to the frankly implausible.
Mechanism (as proposed)
Two properties do the real work. Caffeine is a vasoconstrictor — it temporarily narrows small blood vessels — and it has anti-inflammatory activity. Around the eyes, where skin is thin and pooled blood shows through as bluish shadow and fluid shows as puffiness, that combination produces a genuine, quick, visible effect: less microvascular congestion, a de-puffed, more awake look. This is the basis of its B grade for the vascular / puffiness component of dark circles, and why the dark-circles guide leans on it for that specific type. It has a secondary, more modest antioxidant role.
Where the story breaks down is the body. "Anti-cellulite", "fat-burning", and "lymphatic-drainage" claims ask a topical vasoconstrictor to do things — reduce adipose tissue, move lymph, slim a limb — that it simply does not do. Any visible "firming" from a caffeine body product is a temporary surface-tightening look, not a structural change, which is why those claims sit at D. Bought for the eyes, caffeine is a genuinely useful appearance active; bought for fat, it is marketing.