syringes are disclosed herein incorporating a variety of safety mechanisms for protecting the users from accidental needle stick . in certain embodiments , the syringes incorporate retractable carriages that can be retracted into the syringe barrels by engaging with the plungers . the carriages are configured to receive different needle hubs having any number of needle sizes . in certain other embodiments , the carriages are spring loaded so that as the plungers disengage the carriages from the syringe barrels , the springs automatically retract the needles into the barrels . still in certain other embodiments , needle hubs with spring loaded needles are used with the syringes . the needles are retracted into the barrels when the plungers activate certain mechanisms incorporated into the hubs to thereby release the needles . still yet in certain other embodiments , sheaths are used to cover needle tips .