Will repeatedly injected Botox cause atrophy?

With serial injections of Botox, the muscle being treated will in fact become weaker and smaller, essentially atrophy - to a degree.  It will not lose any function.  As soon as the nerve endings regenerate (typically 3-4 months), the muscle will function normally.  This is actually a benefit of serial injections as the wrinkles formed by the weakened muscle will not be as deep.  

If an area is repeatedly injected over time for many years, then yes it would become atrophied. In this case it would just mean the muscle would be weaker and would either, no longer require Botox or would need much less.

Atrophy is not a long-term effect of Botox injections. Botox causes a temporary muscles weakness but as nerve endings regenerate they gain back movement. 

In the vast majority of cases, Botox will not cause long-term atrophy of the facial muscles. Botox does temporarily block the connection between the nerve ending of the muscle and the muscle itself which causes temporary weakness in the target muscle. However, when the nerve ending regenerates the muscle tends to regain function. Hence the need to repeat treatment every 3-4 months. I hope this information is helpful.Stephen Weber MD, FACSDenver Facial Plastic Surgeon

and botox works not merely by making the muscles less functional, but it's the related calming effect on the tissue above the muscles that is not subject to the repeated stress of pulling and flattening...when the muscles are at rest, then none of this occurs...remember the smoothing effect on the skin is not due to a change in the appearance of the muscles that lie way beneath the surface...it's the skin and other soft tissue above the muscles that demonstrate the effect of the botox actions on the muscles...