A blunt needle plastic cannula for a syringe has a central axial passage terminating in a tip opening axially through one end thereof. Different axial sections along the cannula have respective differential resistances to movement relative to a membrane upon insertion of the cannula into the membrane. The opposite end of the cannula may have a Luer fit or Luer lock or may have integrally formed therewith an adapter for releasably connecting the adapter and cannula to the distal end of the barrel of the syringe. A syringe plunger and the adapter have connective structure whereby the adapter and cannula can be withdrawn into the interior of the barrel. The opposite ends of the barrel are sealed by portions of the plunger.