Every state is a little different in terms of the scope of practice medical assistants may perform. In the state of California, MAs can administer medications such as simple injections, oral, and inhaled medicines-- this must be done after verification by a physician. They are not allowed to inject products such as collagen- because these are highly specialized procedures.
In some states, MAs may be allowed to do sclerotherapy, but just because they can inject, it doesn't mean they should! When properly done, sclerotherapy is safe and effective. But complications can arise including skin ulcers, blood clots, dark pigmentation, etc. In my practice I train many physicians how to do proper sclerotherapy. While it may look easy, it is really harder than it looks-- specific techniques, concentrations, etc are important to maximize results and minimize side effects.
Don't skim on costs by going to an MA. See a vein specialist who will do the treatment themselves.