I own In The Shadows props www.intheshadows.ca. In all unionized productions, an actor will never be punctured with a syringe. Movie prop syringes range from $80-$100 for low end products where the needle will retract, to $400 - $800 where the needle will retract, is spring loaded, and will come back out as the needle is pulled from the arm. Also, the High end have a second system, where liquid can be filled into the needle (blood drawn, or what ever color liquid is coming out of your alien), or liquid injected (clear, Heroin, floating particles, etc). This is done via a 4 foot hose connected to the side of the syringe. At the other end is a vile with a bladder. That is where fluid goes to when injected, or fluid comes from when liquid is drawn from the needle. We make custom props. However, the high end syringes take very specialized materials and craftsmanship to make. So we purchase from trusted suppliers. One of the best effects is having a clear liquid (i.e. heroin) in the syringe. The bladder then has red liquid (blood), and the tube is also full of blood. The syringe is injected (pushed against skin, and spring loaded needle goes into shaft), and a little blood is drawn into the syringe to confirm you hit a vein. This close up is a deal seller on film. Then the syringe is depressed shooting the heroin (into the bladder at the end of the hose). The shot usually changes here, as the specialty syringe is replaced with a real (dulled) syringe (another prop too long to talk about). And with a little movie adhesive, left in the arm/ leg / wherever, as the actor removes the rubber band to get stoned.
Hope this helps. P.S. as a pro in the industry, I would never recommend trying to make your own spring loaded needle. You never want to put an actor at risk.