Active Charcoal has some abrasive ingredients which make it rough for your tooth enamel. A loss of enamel results in higher risk sensitivity and tooth decay. When enamel is removed, a layer of calcified yellow tissue emerges which makes your teeth look paler than before. The adverse reactions of activated charcoal are rare and very infrequent. Do not apply it if you have any cut or open wounds in your mouth. If you face any, please visit your dentist as soon as possible to stop the damage.