A safety device for an injection syringe includes a syringe support, a protective sheath for protecting an injection needle carried by the syringe, the sheath being mounted to slide relative to the syringe support between an injection position in which the needle is uncovered, and a protection position in which the needle is covered, the sheath having a grip surface enabling the device to be gripped by a user and referred to as an initial grip surface, and a return spring for urging the sheath into the protection position. The sheath also includes an extender member for extending the initial grip surface and fitted on the sheath in such a manner as to present a larger grip surface, referred to as an enlarged grip surface.
