According to Paula Begoun's website:

"Benzoyl peroxide negates the effectiveness of most retinoids (i.e., Retin-A, Tazorac) and therefore cannot be used at the same time. To get both benefits, you can use benzoyl peroxide in the morning and the retinoid in the evening. However, Differin (adapalene) has been shown to remain stable and effective when used with benzoyl peroxide (Source: British Journal of Dermatology, Oct. 1998, page 139). "

Just for the record, I've been prescribed Retin-A, tetrinoin and Differin...Differin was much, much too irritating, and really seemed to make my skin worse, even after at least 6 weeks of use. Tetrinoin worked very slowly but was not irritating the first time I tried it 1 year ago, but when I tried to use it again recently (3 months ago), it didn't work at all and was very irritating. I didn't like how shiny Retin-A made my face when I used it...

So I'm just sticking with Dan's regimen and products because they work better than anything I ever tried before