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).
