I have considerable experience using Botox for aesthetic purposes, having started doing so in 1991, eleven years prior to its official FDA-approval for this purpose. Throughout that time, from time to time, I have encountered individuals who had at first responded well to treatment, but who, after two or more prior treatments, demonstrated either no response, weaker response or shorter duration of response.
The medical literature estimates the development of antibodies in the range of 3%-13%. However, it seems that even among those with proven antibody formation few actually demonstrate any significant loss of responsivity to further treatments. This of course does not rule out that some may indeed do so and this fits my own experience in twenty-one years of injecting Botox for dynamic wrinkles.
An increased chance for promoting antibody formation may also be related to the routine practice of bringing patients back after just two weeks to touch up any areas that may not have completely responded to treatment. For this reason, it is currently deemed wiser to bring patients back no sooner than a month following treatment for touch ups.
When there exists no other obvious explanation for loss of response or shortening of duration of response (such as changes in dosing or concentration, etc.), and antibody formation is suspected, a trial of Xeomin, another bolulinum neuromodulator would be reasonable. Xeomin is essentially a naked Botox, stripped of the proteins that come attached to Botox that are believed responsible for triggering the development of antibodies. I have found Xeomin to helpful among the few patients I have had recently in whom I have suspected antibody-related resistance, In such instances I have seen full responsivity restored.