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BACKGROUND The unit equivalence between the two main Botulinum neurotoxin A (BoNTA) preparations, Dysport (Ipsen Ltd., Slough, Berkshire, UK) and BOTOX (Allergan Inc., Irvine, CA), is a matter of discussion. The UK assay used to test Dysport is more sensitive than the U.S. assay used for BOTOX, resulting in a different efficacy per unit in both formulations. Ratios ranging from 6:1 to 1:1 can be found in the literature, but the more recently published literature suggests that 1 unit of BOTOX is equivalent to approximately 2 to 4 units of Dysport (ratio 2:1–4:1).

OBJECTIVE Because the number of BoNTA treatments is constantly increasing, these differences warrant a systematic review of published evidence about the unit equivalence of UK and U.S. formulations.

METHODS The review is based on a detailed literature research in all relevant databases (MEDLINE, PubMed, Cochrane Library, specialist textbooks).

RESULTS The present review supports the recent assumption that dose ratios of less than 3:1 (e.g., 2.5:1 or even 2:1) between Dysport and BOTOX are probably more suitable.

CONCLUSIONS The current evidence is still insufficient, and further investigation of lower dose ratios is recommended.