Urea

Measured on: barrier improvement under twice-daily urea — lower transepidermal water loss, with reduced reactivity to an experimental SLS irritant challenge.

The only urea study with a visit schedule read at three points and separated early. Urea-containing moisturisers (one of them at 10% urea) were applied to normal skin and measured after three applications, at 10 days and at 20 days: TEWL was already lower by day 10, and skin reactivity to sodium lauryl sulphate was reduced after only three applications, before any TEWL fall — so the first measured change lands inside the first fortnight, not at three or four weeks. Two limits on that: the population was normal skin rather than dry or atopic skin, and the endpoints are instrumental, so this is a barrier measurement rather than something the user watches happen. The graded outcome a user would actually see is slower and separate: in 109 atopic dermatitis patients treated for 30 days, a cream with 4% urea plus 4% sodium chloride improved dermatologist-assessed dryness and gave lower TEWL than 20% glycerin — one reading at the end of the course, a genuine ceiling, and at 4% rather than 10%. The humectant lift on a corneometer is faster still but has no dated trial behind it. The keratolytic action the base mentions needs 10-40% urea, above the 2-10% in most consumer moisturisers.

PMID:8932589 (Loden, Arch Dermatol Res 1996, urea-containing moisturisers on normal skin; readings after three applications, 10 days and 20 days; TEWL, capacitance and SLS reactivity) for the timing. PMID:11737814 (Loden et al., Skin Res Technol 2001, 109 atopic dermatitis patients, 30 days, randomised double-blind, 4% urea + 4% NaCl vs 20% glycerin and its placebo, dermatologist-graded dryness plus TEWL and capacitance) for the slower graded-dryness clause.