An injector apparatus comprises a housing (101, 102) for a cartridge or syringe, a plunger (40) for cooperating in use with the cartridge or syringe to express successive doses, a dose setting arrangement (16, 20) to select a dose volume, and a drive mechanism (22, 24) releasable to advance said plunger in respective predetermined increments of magnitude to express said successive doses. The drive mechanism includes a drive spring (22) for imparting movement directly or indirectly to said plunger (40), and said dose setting arrangement comprises a dose setting element (16) moveable in a dose setting routine to define a magnitude of an increment of movement of said plunger for a given dose. The drive mechanism further includes a clutch arrangement (56) operable during said dose setting routine to inhibit forward movement of the plunger and/or to isolate said dose setting element from the force of said drive spring.
