Two patients with isoniazid-induced photosensitive lichenoid eruptions confirmed by photopatch test

Causative agents of drug eruptions are frequently unknown, and skin tests with candidate drugs would be useful before systemic challenge. It remains to be clarified how phostosentive lichenoid drug eruptions are induced, but allergy, including delayed type allergy, has been suggested. Two patients who had taken anti-tuberculous drugs developed a lichenoid drug eruption, primarily on sun-exposed skin. Patch and photopatch tests were performed with each of the ingested drugs (10% in petrolatum). Photopatch tests to isoniazid (INH) were positive. These were confirmed by oral challenge followed by irradiation with UVA. In conclusion, photopatch tests facilitated identification of the causative drug in two patients with photosensitive lichenoid eruptions to INH.