Is Botox applied to the trapezius to reduce its size a procedure that can be (legally/safely) done in the US?

Never heard of that application. In theory, that would work but in practicality, it would be dangerous as the trapezius muscle is a large and very strong muscle such that the amounts of neuromodulator it would require to decrease the muscle bulk would be unsafe due to the risk of causing botulism. I would advise against that.

A large muscle such as the trapezius would require large doses of neurotoxin to effect A change. In my view this may not be safe and may lead to botulism. Hope this helps. Dr. Joseph.

Injection of the trapezius with Botox is not an FDA approved indication. it would probably work but would take a great deal of Botox to get any kind of result. discuss with a board certified neurologist

If it is medically necessary for you to have the trapezius treated,  Botox can definitely be done by either a neurologist or a pain management specialist. Good luck from New York City

Botox is used in small volumes for small muscles (eg face), slightly larger volumes for sweating areas eg armpits and in even larger volumes for larger leg muscles in spasticity. The trapezius is the second largest muscle in the body and I cannot see any benefit to having botox as you would need huge volumes, there will be side effects and you may struggle to maintain your head position!

Botox is often used off-label in the treatments of different conditions. For exemple, it is often used for masseter hypertrophy and slimming of the lower face. It can also be injected at the trapezoid, calf muscle etc., to decrease the size of the muscle and for a slimmer look at the area treated. Please see an expert in your area. All the best, Dr Michon