An injection device of the kind having a main housing (4) is disclosed and includes a drive mechanism (42) to expel selectively a dose of medicament from a medicament cartridge (40) through a needle unit (22) and an electronic control unit to control the drive mechanism (42). The injection device further comprises a prime button (26) which causes a prime signal to be sent to the electronic control unit, the electronic control unit actuating the drive mechanism (42) to expel a predetermined small dose of medicament on receipt of the prime signal. In this way removal of entrapped air from the medicament cartridge (40) is obtained.