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Table 1 Grades of Evidence

From: The treatment of migraines and tension-type headaches with intravenous and oral niacin (nicotinic acid): systematic review of the literature

A Systematic reviews of randomized controlled trials and/or randomized controlled trials.
B Systematic reviews of observational studies and/or high-quality observational studies including cohort and case-control studies.
C Case-series, case-reports and/or poor quality cohort and case-control studies.
D Expert opinion without explicit critical appraisal, or based on physiology, bench research or "first principles."