Topical vitamin C
Strong laboratory rationale, a thin and short human trial base, and a formulation problem that most of the marketing quietly ignores.
Retinoids
The one topical category where the human trial base is deep enough to justify the confidence. Also the one most people abandon before it works.
Niacinamide
The most tolerable active in common use, with a plausible mechanism and a human trial base that is broader than it is deep.
Collagen supplements
One of the fastest growing supplement categories in the UK, resting on a trial base whose funding pattern should give any reader pause.
Vitamin D and immunity
The clearest example on this site of a claim that is well supported for one group of people and unsupported for everybody else.
Zinc and skin healing
A nutrient whose necessity is beyond dispute and whose supplement marketing rests on a claim the evidence does not make.
Polyphenols and photoprotection
The most scientifically interesting weak evidence base on this site. Excellent laboratory work, very little of it demonstrated in people.
Biotin for hair and nails
A real cofactor with a real deficiency state, and a cosmetic claim that reaches well past the people the evidence was drawn from.
Probiotics and the skin gut axis
A genuine and fast moving research field, and a marketplace selling conclusions the research has not yet reached.
This article contains no affiliate links, no sponsored placements and no links to any commercial product, brand, retailer or clinic. Nobody paid for it, nobody previewed it and nobody can have a grade changed. Our editorial policy sets out the single disclosed exception that applies to four archive articles, none of which is this one.
Nothing here is medical advice. Speak to a pharmacist, a GP or a dermatologist about your own circumstances.
Before you read a grade
Three pages explain what the grades mean, how to assess a study yourself, and why an ingredient's origin tells you nothing about whether it is safe.
The appraisals in full
A grade is a summary. These are the appraisals behind them: what each study measured, how long it ran, who paid for it, and the point at which the marketing goes past what was shown.
Topical vitamin C: evidence appraisal
Form by form, with pH, stability and packaging treated as part of the evidence rather than as packaging.
Oral collagen for skin: evidence appraisal
What the randomised trials actually measure, how digestion changes collagen, and where the funding pattern sits.
Oral hyaluronic acid for skin: evidence appraisal
Trial duration, dose and molecular weight, and the limits that apply to the hydration and wrinkle claims.
Topical azelaic acid: evidence appraisal
Acne, rosacea and pigment claims judged separately, with the UK product categories that decide strength.
Biotin for hair and nails: evidence appraisal
A real deficiency state, a cosmetic claim that reaches well past it, and the blood tests biotin distorts.