A pen-type injector is improved so that a user may select a delivery dose of medicinal product from a multidose cartridge. The injector includes a housing. a cartridge containing medicinal product retained within a housing, a piston rod, a dose selecting member to select a dose of medicinal product to be expelled, and a drive member to expel the selected dose of medicinal product. The housing is formed unitary so that the dose selecting member and the drive member are moveably retained within the housing. The injector may include ratchet means associated with the insert to ensure the piston rod only rotates in a single direction through the insert, a button located on the drive sleeve, and clutch means which upon depression of the button prevents rotation between the dose dial sleeve and the drive sleeve.