In the August Q&A on Patreon, Mildred asked: Question pls: Can I use a glycolic toner followed by a Retin A cream" I read a blog that these two needs to be used on alternate days but I can't seem to find that writing any more. 

And Mercedes asked: I would like to know more about using Vitamin A and its derivatives in different products. I know some things are medical grade, but I mean things like retinols and learning which suits which skin types the best.

Vitamin A is an oil soluble molecule that can improve skin barrier function, increase cell proliferation, increase thickening of the skin, and increase collagen production. It can also help increase skin's water retention, and it may be effective in preventing, retarding, or restoring changes associated with the aging process. It is also effective in wound healing. It is the most abundant vitamin in our skin (in the form of ester retinyl palmitate), which is hydrolyzed to form Vitamin A, which is then oxidized to produce retinoic acid (the active form).
There are several forms of Vitamin A - the retinoids - we can use in bath and body products - retinol, retinyl esters (retinyl propionate, retinyl acetate, retinyl palmitate), and retinaldehyde. Each of these is ultimately converted into trans-retinoic acid, the active form of Vitamin A in our skin, but the retinyl esters are less effective than retinol, and are less stable in our products. We can find retinoic acid (tretinoin) in some of our oils.

Vitami...