This is a great question. After sunscreen, tretinoin Retin-A) is the most important part of your overall skincare routine. I have pts everyday who say they can't tolerate a retinoid (Retin-A). They say it turned them fire red and made them dry and their skin peel. This is soooo commonly heard that I know before I hear the answer what happened. The irritation is usually due to a concentration that was too high for their skin. I have been using it for 10 years and still can only tolerate the lowest strength 3 times a week. That's OK. I alternate with something else like glycolic acid cream.