From my experience exploring skincare products containing azelaic acid, I've noticed that the percentage alone doesn't tell the full story about effectiveness. For example, a 15% gel can sometimes outperform a 20% cream because gels often have better penetration thanks to their lightweight and water-based formulations. However, this advantage depends heavily on the specific formulation and the skill of the cosmetic chemist behind the product. In some products I've tried, the 15% gel felt more soothing and absorbed quickly without leaving a residue, which helped with consistent use on my skin. The 20% creams, while having a higher concentration, occasionally felt heavier and less comfortable, potentially creating a barrier that limited ingredient absorption. It’s important to remember that formulation includes all the ingredients used, how they're combined, and their delivery system—not just the active ingredient’s concentration. Additionally, validated clinical testing of these formulations is key to ensuring they deliver real benefits. If you’re considering azelaic acid for hyperpigmentation or acne, don’t just look for the highest percentage; look for reputable brands that emphasize formulation quality and transparency about testing. Personal patch testing is also important, as your skin's response can differ depending on many factors like sensitivity and skin type. This insight has made me more discerning about azelaic acid products and has improved my skincare outcomes by focusing on well-formulated products rather than chasing highest percentages alone.