Erythema multiforme-like eruption induced by risedronate

Risedronate sodium [CAS no. 115436-72-1, P,P′-(1-hydroxy-2-(3-pyridinyl)ethylidene) bis-phosphonic acid sodium] is a bisphosphonate that is used to prevent and treat osteoporosis. Cutaneous adverse drug reactions (CADRs) induced by risedronate are rare [1]; among these, erythema multiforme-like eruption has never been reported.Bisphosphonates are widely used to treat postmenopausal and corticosteroid-related osteoporosis and to prevent the risk of osteoporotic fractures. They are divided in two classes that work differently in killing osteoclast cells: the non-N-containing class (clodronate, etidronate, and tiludronate) and the N-containing class (alendronate, ibandronate, neridronate, olpadronate, pamidronate, risedronate, and zoledronate). Among these, risedronate is considered to be safe and well tolerated [1], and CADRs are relatively rare [1, 3-6], hitherto not including erythema multiforme-like eruption confirmed by a positive patch test reaction (Table 1). Negative patch test results with other two bisphosphonates belonging to the same N-containing class (alendronate and ibandronate) showed no cross-reactivity to these compounds.