Are multiple sessions of Botox injections needed for slimming the masseter muscle before results are noticeable?

You probably just need additional units. As with other areas of the face, there is a range of number of units that is effective for a given patient. I typically start with 25units per side but this can certainly be increased for the desired effect. If you didn't get before and after photos you should, as results can sometimes be difficult to see. Hope that helps and best wishes, Dr. Donath 

Yes, it can take multiple treatments of Botox to slim the masseter. Keep in mind, Botox results last 3-4 months. Radifequency treatments can help improve results. See an expert in facial contouring for a formal evaluation. Best, Dr. Emer

Thanks for this question.The doses for Masseter injection usually vary between 15-30 units per side. It's very unusual not to see a significant change in facial shape after the dose that you have received.It would not be unreasonable to try  it once more with the same dose. I would make sure that you have good photos to objectively asses the result.