An injection device comprises a housing for receiving a syringe or cartridge for containing a medicament. It further comprises a rotary driveshaft 24 mounted for rotation relative to said housing and an elongate coiled torsion spring 38 having respective formations at opposite ends thereof. A first formation 50 is anchored in a seat 56 on said rotary drive shaft 24 and a second formation 48 being anchored in a seat 58 on a reaction component 14. In use relative rotation of said rotary driveshaft 24 and said reaction component 14 in one angular direction strains said torsion drive spring 38, and release of said strained torsion drive spring 38 causes expression of medicament from said syringe or cartridge. The torsion spring 38 includes at least a region (52) of open coil form whereby during assembly of said injection device, the torsion spring 38 may be longitudinally compressed and then rotated, thereby urging the end formations 48, 50 of the torsion drive spring into engagement with said respective seats (58, 56).
