This case report documents a patient who developed persistent methemoglobinemia from unsupervised use of a topical dapsone-adapalene combination product, highlighting the risk of systemic absorption and adverse effects from unmonitored application of this dermatological agent. The patient’s methemoglobinemia was treated with methylene blue, demonstrating its effectiveness in reversing dapsone-induced oxidative damage to hemoglobin. This is a human clinical case report (low evidence level) that illustrates an important safety concern with topical dapsone use.