Retinol

• Measured on: investigator-graded photoaging parameters versus vehicle - lines and wrinkles, pigmentation, elasticity, firmness and overall photodamage on a 0-9 scale.
• Tucker-Samaras et al. tested a stabilised 0.1% retinol moisturiser once daily against its vehicle in a double-blind split-face study of women with moderate facial photodamage (36 on retinol, 28 on vehicle), grading at baseline, week 4 and week 8.
• At week 8 retinol was significantly better than vehicle on lines and wrinkles, pigmentation, elasticity, firmness and overall photodamage, and the paper records that many of those differences were already significant at the week-4 visit and improved progressively to week 8.
• The wait is therefore 4-8 weeks, with 8 weeks the ceiling for the full set of parameters; the paper does not say which individual parameters cleared significance at week 4, so the earlier figure is not claimed for wrinkles specifically.
• The 24-week 0.4% study usually quoted next to it is not a longer answer to the same question: it treated the inner upper arms of subjects with a mean age of 87 three times weekly and reported at 24 weeks.
• This is an industry study (Johnson & Johnson) with modest numbers, and 0.1% sits at the bottom of the 0.1-1% consumer range.
• Acne is a separate and slower wait, and the dryness and flaking most users notice in weeks 1-2 is irritation, not an outcome.

Tucker-Samaras et al., J Drugs Dermatol 2009;8(10):932-6, 'A stabilized 0.1% retinol facial moisturizer improves the appearance of photodamaged skin in an eight-week, double-blind, vehicle controlled study', PMID:19852122 - abstract read: evaluations at baseline, weeks 4 and 8; significance versus vehicle at week 8, many differences already significant at week 4. Kafi et al., Arch Dermatol 2007;143(5):606-12, PMID:17515510 (0.4% retinol, three times weekly, 24 weeks, inner arms, mean age 87).