I have used it on and off for several months last year. Gave it up because Retin side effects is very sensitive. For example, Retin-A is great because it literally peels away layers of your skin. It's hard to put it in words, but this is literally how it helps minoxidil absorbtion. (ie. less surface skin = better penetration) However, if you overdose then your area of application easily becomes redden and sensitive to touch. It's hard to utilize this product on your face if you are exposed to the public daily. You neeed to read up on previous posts and experiment yourself to find the right dosing for you.
I usually apply it every other day or wait two consecutive days during the night, "after" minoxidil dries. The problem is you can only apply so many topicals at once, and I gave up on Retin because imagine applying minoxidil liquid to a newly healed wound over large areas of your skin. Imagine the sharp sensitive pain when your skin come into exposure with minoxidil's alchohol content. This is okay when you only have to do it once a week, but when this becomes a ritual and you are constantly battling with removing dead scalp skin /dandruff during shampooing. It just becamse a hassle that I didn't want to bother with. Anyhow, hope this helps...