The incidence of anaphylactic reaction after the long-term use of abatacept has not been reported until now. Herein, we present a case of rheumatoid arthritis (RA) in which the patient experienced an anaphylactic reaction one year after initiation of treatment with abatacept. A 75-year-old woman visited our hospital with symptoms of bilateral knee pain and swelling. She was initially treated with methotrexate (6 mg/week increased to 8 mg/week). Two months later, because of inadequate response,

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... elf-injections of abatacept (subcutaneous; 125 mg every two weeks) were prescribed. However, 6 months later, because of frequent stomatitis, the methotrexate dose was decreased to 6 mg/week, which resulted in worsening of RA. We changed the route of abatacept administration from subcutaneous injection to intravenous infusion (500 mg/month as a drip). After 30 min of starting the drip, the patient experienced itchiness and drop in vital signs, which were managed using methylprednisolone (2 doses, 125 mg each), dopamine hydrochloride (8 mg/h), and oxygen therapy (flow decreased from 3 L/min to 1 L/min). Wheals and redness were treated with oral antihistamines. Six hours after the onset of the anaphylactic reaction, the vital signs were stabilized. On the subsequent day, the patient's general state was confirmed to be normal. One month later, etanercept (25 mg) treatment was initiated. The patient is currently in remission. We recommend caution when changing the route of administration and dosage of abatacept in anti-cyclic citrullinated peptide antibody-positive patients or those with a history of mild infusion-related reaction.