In my experience, azelaic acid treats inflammatory acne better than non-inflammatory acne (whiteheads and blackheads). I was still getting a few stubborn whiteheads, so I added adapalene into my routine. My line of reasoning was that adapalene’s strengths are the inverse, in that it’s known to treat whiteheads and blackheads better than inflammatory acne. Also, while Differin, like retinoids in general, is pretty lacklustre as a monotherapy, it enhances the effectiveness of an ingredient used in combination with it (Epiduo, the combination of adapalene and BP, is a good example). So basically I mix my own variation of Epiduo tailored for adult skin, replacing the BP with azelaic acid.