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The authors followed 532 consecutive patients with episodic migraine (<15 days/month) for 1 year. Sixty-four patients (14%) developed chronic headache (>/=15 days/month). The odds ratios for developing CH were 20.1 (95% CI 5.7 to 71.5) comparing patients with a "critical" (10 to 14 days/month) vs "low" (0 to 4 days/month) and 6.2 (95% CI 1.7 to 26.6) in patients with an "intermediate" (6 to 9 days/month) vs "low" headache frequency and 19.4 (95% CI 8.7 to 43.2) comparing patients with and without medication overuse.
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Katsarava et al. (2004) studied this question.
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