Funny you should ask....I just asked Dr. Lee about adding Retin-A to my regimen. Here's a portion of his reply.
Although I sell topicals
with Retin-A, I don't routinely recommend its use anymore. Retinoic acid
(Retin-A) works as a chemical peel and can cause mild irritation and flaking for the first 10 days or so. The reaction seems to be more severe on fair complexions. The addition of retinoic acid is primarily to allow for better absorption of the minoxidil. This was very important when 2% minoxidil solutions were the only concentrations available. Now that 5% and higher concentrations are available, the addition of retinoic acid is not as important. In fact, the undesirable side effects of retinoic acid use (dryness, redness, flaking, increased sensitivity to sun damage, etc.) may not counterbalance its
'benefits'. If you have a sensitive scalp, you may not want to use it.