An apparatus for use with a syringe to provide a safety syringe, the apparatus comprising: a sheath (100; 300; 500; 700; 900; 1100; 1300) deployable for at least partially covering a needle of the syringe; a sheath actuator (102; 302; 502; 702; 902; 1102; 1302) for deploying the sheath; and a ratioed mechanism (104; 304; 504; 704; 904; 1104; 1304) linking the sheath and the sheath actuator and configured such that a magnitude of movement of the sheath is greater than a magnitude of movement of the sheath actuator during deployment of the sheath.一種可與一注射器一起使用以提供一安全注射器的裝置,該裝置包含:一套鞘可展開用以至少部份地掩蔽該注射器之一針;一套鞘致動器用以展開該套鞘;及一比率式機構連結著該套鞘和該套鞘致動器,並構製成當該套鞘展開時會使該套鞘之一移動量值係大於該套鞘致動器之一移動量值。