Botox should not be used to decrease size of large muscles
The sternocleidomastoid muscle helps turn the head. The two muscles working together regulate the position of the face - straight or off to the side. Treating it with Botox can potentially cause a decreased function of one muscle, causing a deviation of the head from its central position.
I must say I wonder about the sudden number of questions about using Botox to decrease the size of large muscles. And as many questions as there are about this topic, there are many more answers talking about the danger and the inadvisability of performing such a procedure.
Using Botox on large muscles would require large quantities of Botox which can potentially be dangerous or even life-threatening. Also, it may induce antibody formation to Botox, and since Botox use is needed every 2-3 months to maintain results, it will be useless with subsequent injections. Please don't do it.
Using Botox to Shrink Muscles for Aesthetic Purposes
Over the years, Botox has taken on many new uses that are quite controversial. Botox has been used to shrink certain facial muscles to change the hard, angular shape of the jaw line. Also, in certain cultures, Botox has been used to decrease the size of the calf muscles to give a “more feminine” appearance. Using a neurotoxin such as Botox to weaken muscles that are necessary for everyday use to function in our world, because of perceived cosmetic imperfections, seems crazy to me. No reputable physician should provide these treatments.
Botox for enlarged sternocleidomastoid muscle
Although Botox could theoretically reduce the size of the sternocleidomastoid muscle, it is not advised, since this muscle is important for head control and movement. You may want to consult with a neurologist to discuss possible reasons for the enlargement of this muscle.
Yes it could do this through the process of disuse atrophy.
Unfortunately the sternocleidomastoid muscle is one of the critical strap muscles of the neck. Weakening this muscle with BOTOX could make it very difficult to hold up the head. For this reason, treatment of this muscle for cosmetic reasons is not a good idea.
BOTOX® is used to relax the sternocleidomastoid, but not to shrink it
BOTOX® is almost always used to relax various muscles [and rarely used to completely paralyse certain muscles].
Certain individuals who suffer from a condition called torticollis ["neck twisting"] benefit from relaxation of the sternocleidomastoid [and sometimes other muscles] after treatment with BOTOX®. After treatment with BOTOX® the sternocleidomastoid might not contract so hard, and so would not stand out so much, but it would be unlikely to shrink.
The same effect can be seen when the flat muscles ["platysmal bands"] of the anterior neck are relaxed with BOTOX®: they do not jump out as much when the individual speaks or makes certain movements, so the neck looks smoother and more attractive, but the muscles do not actually shrink.
In cases where there is excessive growth of a muscle [for example, the masseter muscle, in particular in people with bruxism who grind their teeth at night] find that relaxation of the masseter with BOTOX® controls both their bruxism and the associated tempormandibular joint ["TMJ"] pain, and are pleased that the size of their masseters gradually comes down to normal, so that they do not have a chipmunk-like appearance.
Botox is NOT to be used to shrink muscle mass in the body.
I have noticed an increase recently in people asking about using Botox to shrink normal body muscles because they don't like the look of the muscle. Botox paralyzes muscles, and the muscles of the body are used for structural support, normal movement, and safety (reflexes require normal muscle function to protect you from unexpected injury or danger).
Botox is NOT to be used to shrink otherwise normal body muscles. If there is a medical problem with muscle spasm, see a neurologist to get the appropriate amount of MEDICALLY NECESSARY but safe Botox. Do not go to any non-MD providers for muscle issues with Botox. That would be extremely dangerous.
Botox can relax sternocleidomastoid muscle
If there is a neurologically functional problem such as a twisted neck that becomes painful and the spasm can't be broken with other treatments, than I sould suggest someone with this problem to see a neurologist who could use Botox to relax the problem muscle. It can affect head position and inhibit important movements.