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 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 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 . <EOS>