Biologics have revolutionized the treatment of rheumatoid arthritis (RA). However certain amount of the patients cannot achieve goal of therapy. Recently, compounds targeting the intracellular kinase, Janus kinase (JAK) have demonstrated therapeutic effects resembling biologics. Tofacitinib is the only JAK inhibitor approved for RA and during the clinical trial, increased events of herpes zoster (HZ) was observed. Incidence rate was twice as much as patients treated with conventional anti-rheumatic drug and was especially increased in Japan that was four times as much. The risk factors were age and glucocorticoid that is identical to that of common RA patients and there was nothing specific for tofacitinib. Mechanism of increased incidence of HZ and the difference in ethnicity remains unknown. Analysis of clinical trials have identified that HZ do not correlate with further adverse events. Therefore, it is extremely important to accumulate clinical data with considerable amount of patients with long term follow up including the post marketing surveillance in Japan to reveal the significance of increased HZ in RA patients.
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