Absolutely. It came out of you, why would it be unsanitary to put it back in you??? I mean unless you are taking the pin out then squirting all the stuff in the syringe into a pile of shit and then sucking it back up and then injecting it back into yourself, then it is unsanitary.
It is just like air, I always leave a little air in the syringe and inject until the air goes into the needle that way I get all the gear into myself. Injecting a little bit of air is not going to hurt you.