While Dazit M (desloratadine and montelukast) can occasionally cause mild dryness, it is highly unlikely to directly cause a dark brown patch. Instead, this dryness and subsequent dark patch?known as post-inflammatory hyperpigmentation?are likely due to your preexisting flaky rash or acne becoming irritated. When sensitive facial skin becomes inflamed or very dry, it can heal leaving behind darker pigmentation. Please use a gentle, fragrance-free moisturizer and avoid applying harsh acne treatments around your mouth. I highly recommend consulting your dermatologist, who can safely evaluate your rash and help fade the dark patch.