A dose setting mechanism (102) having a locking mechanism (200 300 400) for preventing selection of a dose in a drug delivery device (100) is disclosed. The dose setting mechanism (102) includes a locking mechanism (200 300 400) that prevents movement of the dose setting mechanism (102). When a correct cartridge (120) is inserted into the drug delivery device (100) the locking mechanism (200 300 400) unlocks so as to allow movement of the dose setting mechanism (102). In one arrangement the locking mechanism (200 300 400) prevents rotation of the dose setting mechanism (102).
