A very popular treatment for patients who suffer with temporomandibular dysfunction (TMJ). Where the masseter muscle, just above the jaw, is overly clenched and may become sore. Over time this leads to hypertrophy (increase in size of the muscle). 
The other common indication for masseter anti-wrinkle injections is for hypertrophy of the masseter muscle, without associated TMJ. The injections are used to decrease the exertion of the masseter muscle, leading to a slimmer appearance.