Background: Alopecia areata (AA) and irritable bowel syndrome (IBS) are two distinct diseases that share a similar pathophysiology; however, the relationship between these two diseases is unknown. This study aimed to investigate the bidirectional relationship between AA and IBS. Methods: Participants were recruited from the National Health Insurance Research Database in Taiwan. We included 5446 patients with AA and 21 784 matched controls to assess the risk of IBS, and 56 429 patients with IBS and 225 716 matched controls to assess the risk of AA. The Cox proportional-hazards regression model was used to calculate the adjusted hazard ratio (aHR). Results: After adjustment for potential confounders, patients with AA had an aHR of 5.20 [95% confidence interval (CI) 3.97–6.82] for IBS in comparison with the controls. Furthermore, compared with the controls, IBS patients had an aHR of 5.38 (95% CI 3.95–7.34) for AA. Conclusion: AA is bidirectionally associated with IBS. Further investigation is needed to elucidate the shared pathogenesis underlying these two diseases.
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