3 weeks post op, how long until masseter Botox starts to show?

Thank you for your question. Botox in the masseter region usually takes longer than in the standard areas to take effect. I tend to use anywhere from 15 units (conservative treatment) to 50 units (for someone with really strong masseters) on each side to get the desired result. It may start within the first week but I tell my patients to wait three weeks before doing a more thorough evaluation. The treatment typically lasts longer than the forehead/crows feet/frown regions. It is also a gradual onset of the return of masseter function. I have tried the masseter injections myself and space the treatments around 4 months apart, but use smaller doses to maintain the effect. I normally know the effects are wearing down when I start to clench my teeth more heavily at night when asleep. Once you stop using Botox, the masseter muscles will return to their original baseline when the effects of Botox wears off completely.It is hard to know why you are not seeing results that you are expecting without being to evaluate before/after pictures and know the injection technique and dosing used by your injector.  You may want to revisit your injector or also seek a second opinion from a more experienced injector.

Considering you have had 2 injections, wait 4 weeks until you evaluate your results. It doesn't hurt to visit your injector and have your questions answered. Botox takes time to show results. Initially you may need more Botox depending how strong your muscle is. 

The masseter is one of 4 muscles involved in chewing, and it is a very large, bulky muscle.  Botox will effectively relax this muscle in about a week, however it does take time for the muscle to atrophy and shrink.  Over the course of the first month, after an effective treatment, you should notice a gradual tapering of the lower third of your face.  Especially for first-timers, an additional treatment may be necessary 2-4 weeks after the first treatment to fully relax the muscle.