No, micro-current uses power settings that are very low and as such should have no effect on the previously treated area(s) however it's always best to ask the plastic and cosmetic surgeon that did the Ulthera treatment.
Will Micro Current Procedures Interfere with the Ultherapy That I Had 3 Weeks Ago?
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No need for microcurrent after Ultherapy
At 3 weeks you will not have seen much of the effects of Ulthera. There isn't much data to support the effectiveness of microcurrent treatment other than on a very temporary basis so why not just wait for the results of Ulthera?
Ulthera and micro-current procedures
All current-related procedures work primarily on the muscle while Ulthera works of subcutaneous tissue/ fascia. I don't think anyone can answer your question with 100% certainty, but I don't believe there would be an interference between two procedures. However, why won't you then repeat Ulthera if you are not fully satisfied and want some adjunctive procedure done?