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Background: There is a growing awareness of constipation being a side effect of anti-calcitonin gene-related peptide (anti-CGRP) monoclonal antibodies (mAbs). This study aims to assess constipation as a side effect and explore its potential role as a predictor of treatment response in chronic migraine (CM) patients treated with anti-CGRP mAbs.

Methods: We conducted a retrospective analysis of patients with CM treated with anti-CGRP mAbs between January 2019 and December 2022 at a single center. Data on patient demographics, migraine characteristics, and constipation were reviewed.

Results: Among 317 patients, 192 received erenumab, 94 received fremanezumab, and 31 received galcanezumab. Constipation was significantly more common with erenumab (51.5%) compared to fremanezumab (4.2%) and galcanezumab (12.9%). Among erenumab patients, 24.4% switched due to constipation, while 19.2% switched due to lack/loss of effectiveness. Patients who switched treatment experienced milder symptoms (p < 0.0001) without compromising effectiveness, even when switching due to lack/loss of effectiveness (p = 0.0068). Importantly, severe constipation rates were higher in non-responders (p = 0.036).

Conclusion: Constipation is a notable side effect of anti-CGRP mAbs, particularly with erenumab in CM. Side effect monitoring is consequently crucial. Our findings suggest a potential link between poor treatment response and constipation, warranting further research into the underlying mechanisms.