Cross sensitivity between fenticlor and bithionol has been shown in seven cases. The positive photopatch test to bithionol depended upon exposure to an increased quantity of sunlight compared to that needed to show positives to multifungin and tribromosalicylanilide. It is postulated that bithionol breaks down to fenticlor on exposure to ultraviolet light. Two cases of allergy to fenticlor are described because of their differing clinical responses. There is evidence that fenticlor can combine with skin protein, that it can be absorbed through skin into the general circulation and, in this way, act in a similar fashion to a drug given internally.
Burry JN. Cross Sensitivity Between Fenticlor and Bithionol. Arch Dermatol. 1968;97(5):497-502. doi:10.1001/archderm.1968.01610110005001