BACKGROUND: The efficacy and safety of OnabotulinumtoxinA (BOTOX®) in adults with chronic migraine (CM) were demonstrated in the PREEMPT program. However, the dosage used in this study was flexible from 155 U to 195 U at the physician's discretion. Therefore, the objective of this prospective study was to compare the efficacy and safety of OnabotulinumtoxinA 195 U vs. 155 U for the treatment of CM and medication overuse headache (MOH) during a 2-year period. METHODS: We prospectively evaluated the mean reduction in headache days, migraine days, acute pain medication intake days and Headache Impact Test (HIT)-6 score in 172 patients injected with OnabotulinumtoxinA 195 U. Successively, we compared the efficacy measures with data of 155 patients injected with OnabotulinumtoxinA 155 U and followed up for 2 years. All patients were affected by CM and MOH, and failed one or more previous detoxification and preventative therapies. RESULTS: Both OnabotulinumtoxinA 195 U and 155 U reduced significantly the number of headache and migraine days, acute pain medication intake days and HIT-6 score, when compared with the baseline measures. Nevertheless, OnabotulinumtoxinA 195 U proved to be superior of 155 U in all efficacy measures since the first injection and for all the 2 years of treatment, with the exception of the reduction in pain medication intake days that resulted significantly larger with 195 U only after the 4th injection. The safety and tolerability of the two doses were similar and treatment related adverse events were transient and mild-moderate. CONCLUSIONS: This study represents the largest and longest post-marketing studies of doses comparison with OnabotulinumtoxinA in a real-life clinical setting. Here, we demonstrate the superior efficacy of OnabotulinumtoxinA 195 U compared to 155 U in CM patients with MOH during a 2-year treatment period with similar safety and tolerability profile.