Azelaic acid is effective for mild to moderate acne when applied topically at a 20% concentration. Treatment twice daily for six months is necessary, and is as effective as topical benzoyl peroxide 5%, isotretinoin 0.05%, and erythromycin 2%. Azelaic acid is an effective acne treatment due to its ability to reduce skin cell accumulation in the follicle and its antibacterial and anti-inflammatory properties. It has a slight skin-lightening effect due to its ability to inhibit melanin synthesis. Therefore, it is useful in treating individuals with acne who are also affected by post-inflammatory hyperpigmentation. Azelaic acid may cause skin irritation. It is less effective and more expensive than retinoids. Azelaic acid also led to worse treatment response when compared to benzoyl peroxide. When compared to tretinoin, azelaic acid makes little or no treatment response.