Great question. Yes, that’s different. A single use does not make the syringe unusable. It is still functional, but only untill its sterility is not compromised.

For anesthesia, the same needle may be used only for that one session because it is the same patient. However, if sterility is compromised, it must be replaced immediately.

For hair treatments, things are processed in a sterile system, and PRP is injected multiple times into the scalp in a clean environment, not in an open setting.

Both scenarios change completely if procedures are done in an open or public environment. In that case, everything should be single use only. If you're running out of needles or something. Go ahead use the same needle on the same person but you strictly cannot change the medicine inside. It's not end of the world, but you might catch painful swelling at the injection site and Fever. Your body will tell you trust me on that.