Why would only 1/2 a filler syringe be used?

thank you for your question. there is only a very small amount of fillers that come in .5mls. if the packaging was already open then there is a possiblity it was used on someone else or to do something else. We do not split syringes in our office although the risk is minimal when the needle is changed there is still a risk when sharing syringes especially since your provider should be aspirating while injecting.

There are sterile dividers that may help one to safely TRANSFER a filler into another sterile syringe BEFORE IT WAS USED. So yes, it may potentially be used for another patient.
Of course, you can not SHARE the same syringe with just changing needles after one-half was used on one patient.
As soon as you did not see if the provider TRANSFERRED it or just changed the needle, there would always be a question left. I hope you did the procedure with the provider who can legally perform the procedure (usually it is a physician, RN, or PA depending on a state) so that you can address it if a problem arises.
The filler may be stored, so I personally prefer to charge for the full syringe and keep the remnants for touch-ups if they desired or offer to inject another area the same day.
Hopefully, that helps.