An injector apparatus for use with a cartridge or syringe to deliver a plurality of doses therefrom, the injector apparatus comprising: a housing (10 1, 10 2) for the cartridge or syringe; a plunger for cooperating with the cartridge or syringe to express successive doses; and a drive mechanism (22, 24) energised by a torsion spring; wherein: the drive mechanism is releasable to advance said plunger in predetermined increments as determined by adjustment of a dose setting element, and the injector apparatus further comprises a rotary re-energising element for re-energising said torsion spring independently of movement of said dose setting element, characterised in that: the rotary re-energising element comprises a circumferential groove with two ratchet teeth in the base of the groove; the housing comprises an annular rib located in the groove and engaged with the ratchet teeth to allow rotation of the rotary re-energising element about the longitudinal axis of the housing in only one direction; the torsion spring is anchored in a slot on the rear end of the rotary re-energising element; and rotation of the rotary re-energising element in the direction permitted by the ratchet teeth re-energises said torsion spring. a housing (10 1, 10 2) for the cartridge or syringe; a plunger for cooperating with the cartridge or syringe to express successive doses; and a drive mechanism (22, 24) energised by a torsion spring; wherein: the drive mechanism is releasable to advance said plunger in predetermined increments as determined by adjustment of a dose setting element, and the injector apparatus further comprises a rotary re-energising element for re-energising said torsion spring independently of movement of said dose setting element, characterised in that: the rotary re-energising element comprises a circumferential groove with two ratchet teeth in the base of the groove; the housing comprises an annular rib located in the groove and engaged with the ratchet teeth to allow rotation of the rotary re-energising element about the longitudinal axis of the housing in only one direction; the torsion spring is anchored in a slot on the rear end of the rotary re-energising element; and rotation of the rotary re-energising element in the direction permitted by the ratchet teeth re-energises said torsion spring. the drive mechanism is releasable to advance said plunger in predetermined increments as determined by adjustment of a dose setting element, and the injector apparatus further comprises a rotary re-energising element for re-energising said torsion spring independently of movement of said dose setting element, characterised in that: the rotary re-energising element comprises a circumferential groove with two ratchet teeth in the base of the groove; the housing comprises an annular rib located in the groove and engaged with the ratchet teeth to allow rotation of the rotary re-energising element about the longitudinal axis of the housing in only one direction; the torsion spring is anchored in a slot on the rear end of the rotary re-energising element; and rotation of the rotary re-energising element in the direction permitted by the ratchet teeth re-energises said torsion spring. the rotary re-energising element comprises a circumferential groove with two ratchet teeth in the base of the groove; the housing comprises an annular rib located in the groove and engaged with the ratchet teeth to allow rotation of the rotary re-energising element about the longitudinal axis of the housing in only one direction; the torsion spring is anchored in a slot on the rear end of the rotary re-energising element; and rotation of the rotary re-energising element in the direction permitted by the ratchet teeth re-energises said torsion spring.