Can Botox in the Masseter Be Permanent and How Is This Muscle Different Than Others?

This is not true. The masseter muscle reacts to Botox in pretty much in the same way as any other muscle. Results last a few months but can last for up to a year depending on lifestyle habits. Touch-up injections may be required twice a year to maintain results.

The masseter muscle is not different than other muscles and does not permanently atrophy. However, as it is used for chewing and clenching  can make it a larger mass of muscle from "exercise". Therefore it may take a larger number of units to relax this muscle than other muscles that are injected for cosmetic indications. As Dr. Andrew Blitzer, a NYC expert in using Botulinum toxin for TMJ and enlarged masseter muscles, says, the muscle  must be exercised to regain its girth, once it is no longer under Botox's effect.  The masseter muscle may take longer than smaller muscles to regain its size as the atrophy is dependent on the number of units of Botox injected. There is no knowledge of permanent atrophy of the masseter muscle.”

The information provided in Dr. Shelton's answer is for educational purposes only and is not intended to constitute medical advice.  The information provided should not be relied upon as a substitute for consultations with a qualified health professional who may be familiar with your individual medical needs.

Botox injected into the masseter(s) muscles is not a permanent fix. It does weaken and atrophy the muscle, causing decreased size and a desirable facial contour for some. It requires follow up every 3 months or so (until you achieve desired results), then maintenance beyond that. Botox works the same on all facial muscles.

The masseter is a muscle of mastication, meaning it helps move your mandible (jaw) for chewing.  Compared to the mimetic (facial expression) muscles that are typically injected with neuromodulators, like the orbicularis, corrugator, procerus, and frontalis, the masseter is a far bulkier muscle.  Injections must be performed deep into the body of the muscle at multiple sites to be effective.  Just as in other areas, you will not achieve permanent dystrophy but rather see a gradual return of function over 3-5 months.  

Botox works well on the masseter muscles to slim the lower third of the face, and it also helps reduce teeth grinding/jaw clenching during the night.  The effect of botox is the same for the masseter, as compared to any other.  Botox works by temporarily weakening the muscle.  The duration of the effect depends on how much is injected and how big the muscle is.