This is the category with the widest gap between what a product claims and what has been demonstrated. Much of that gap is legal rather than dishonest: cosmetics are regulated on safety, not efficacy, so a claim only has to avoid being a drug claim.
The analyses here separate the two. Where there is published evidence for an active ingredient at a stated concentration, we say so and cite it. Where the evidence is a manufacturer-run consumer perception study, we say that too, because "87% of women agreed" means 87% of a small panel answered a question, not that anything measurable happened.
We also work out cost per use rather than cost per bottle, because that is the number that decides whether a routine survives contact with a real month.