A safety device for an injection syringe including a syringe support, a protective sheath for protecting an injection needle carried by the injection syringe, the protective 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 protective sheath having an initial grip surface enabling the device to be gripped by a user, the initial grip surface being a projection that extends outward from the protective sheath, 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, being a separate piece from the protective sheath, attached to the protective sheath in such a manner as to present an enlarged grip surface, larger than the initial grip surface.