Botox causes muscle atrophy over long term use, is this true?

I think its important to separate a few things here, to be clear.  Muscle Atrophy you can see and that you can not see. Botox works by preventing the muscles from contracting. If you think about any muscle in the body, if you don't use it, you loose it. If you had a cast on your leg, after 6 weeks when the cast comes off, your cast leg will have muscle atrophy compared to the healthy leg. However, with physio and physical activity, the muscle comes back.  We also use Botox sometimes to help with this, like in the Masseter Muscle of the jaw. The same is true for Botox. The mechanism of Botox is to prevent muscle motion and thus by its very nature, you are going to get atrophy of the muscle (or, muscle volume loss). This can be desirable in some people as that means you may need lower doses of Botox in the future as there is less muscle. However, the muscles that control facial expression are so small and thin, muscle loss doesn't really show up. However, to say it doesn't cause atrophy isn't true, but it likely doesn't cause any 'visible' atrophy. Dr James Bonaparte

The only time Botox creates the reverse effect and makes a patient look older is when it has been over-injected or injected into the wrong muscle. This should never happen if you find an expert injector.
While Botox does relax muscles and, over time, they may lose tone and therefore require less treatment to stop them from contracting fully, they do not actually atrophy significantly. I have many patients who have been using Botox for decades and it still continues to rejuvenate their faces. I hope this helps.

Botox is a neuromodulator that relaxes dynamic muscles so that they stop pressing the skin into wrinkles and creases. The operative word here is “relaxes.” We give just enough Botox to let the skin smooth out, but not enough to paralyze the muscle to the point where it would atrophy from lack of use. While over the long term, you may need less Botox because the muscle no longer can contract as strongly as it did, this is a good thing. And while the contractions aren't strong enough to reinforce a wrinkle, they are strong enough to keep the muscle agile. While it may be slightly smaller over the long term, it will not droop or cause an aging look. Please consult a board-certified dermatologist who is an expert in using Botox over the long term to manage aging.