We report a rare case of a patient who was referred to the Department of Oral medicine in Zagreb, Croatia. The patient was 20 years old, otherwise healthy and not taking any medication. She presented with irregular erosions partially covered with pseudomembranes that involved both lips and retrocomissural mucosa. Discrete erosion was also noticed on her lower lingual gingiva in the area 42. She reported a propolis solution self-medication for treatment of recurrent aphthous ulcers. After ten days of propolis application, lip and oral lesions developed. Patch test to propolis was proven. We highlight the fact that some folk medicine medications, such as propolis, although being known for many decades to be helpful in various conditions, in some individuals might lead to unwanted side-effects due to its antigenic potential. Additionally, every colleague, during the differential diagnosis of the oral lesions must bear in mind unwanted reactions to folk medicine products.