Methods: We reviewed the charts of 28 patients (17 with benign essential blepharospasm and 11 with hemifacial spasm) seen between 1989 and 1994 by one ophthalmologist at the botulinum clinic at a university-affiliated hospital in Edmonton. All patients had had at least 6 botulinum A toxin treatments; 17 had had 12 or more treatments, and 7 had had 20 or more treatments. A follow-up data collection form was used to record duration of effect, dosage and side effects. We analysed the data on duration of effect and dosage versus treatment number for individual patients and for the overall group using linear regression analysis.

Results: There was no clinically or statistically significant change in duration of effect over time (mean slope 0.024 [95% confidence interval (CI) –0.218 to 0.266]). A positive mean slope of 0.677 (95% CI 0.252 to 1.102) was observed for the change in dosage over time. This trend was most likely related to the practitioner’s habit of using half the normal maintenance dosage for the initial injection. When we eliminated the data for the first injection and recalculated the dosage requirement over time, the mean slope for the change in dosage was 0.321 (95% CI –0.084 to 0.726), confirming stable dosage requirements over time.

Interpretation: The results suggest that patients can be reassured that duration of benefit and dosage requirements will likely remain stable over time for at least their first 20 treatments.