When Skin Reactions Interrupt Bone Therapy: Severe Cutaneous Adverse Reaction to Romosozumab Leading to Treatment Discontinuation.

Background: Romosozumab is a sclerostin-inhibiting monoclonal antibody that is effective and safe for anabolic treatment in patients with osteoporosis. Its main adverse effects are local; the severity of these injection-site reactions in clinical trials was generally mild. Case report: We present a case of a 71-year-old Colombian woman with osteoporosis at very high risk of fractures with no relevant history of drug allergies. Bone-forming management was indicated for 12 months with romosozumab after ruling out contraindications and alterations in bone mineral metabolism that would prevent its use. After the first dose of romosozumab 210 mg subcutaneously in the abdomen, she presented a severe local cutaneous adverse reaction, she developed two asymmetric erythematous-oedematous circinate plaques with regular and well-defined borders with a perilesional whitish halo associated with local heat in the umbilical region, which forced the discontinuation of this medication after a joint evaluation with endocrinology, dermatology and allergology. Management was rotated to teriparatide with good tolerance. During follow-up, complete improvement was achieved slowly and progressively, 20 days after the application of romosozumab, without any other related adverse reactions. Conclusion: This case documents a severe cutaneous reaction at the romosozumab application site, with an atypical presentation due to its extent and intensity, which led to treatment discontinuation. Although these reactions are usually mild, their early recognition is crucial to ensure proper management and avoid complications that could compromise treatment continuity. LEARNING POINTS: Severe inflammatory skin reactions can occur at the romosozumab injection site, even in patients without a relevant allergic history.The detection of moderate or severe local inflammatory reactions allows for an adequate assessment of the risk-benefit balance of romosozumab treatment, which is essential for deciding whether to discontinue treatment and redirect bone-forming therapy.