A lock for a dosing mechanism of an injection device, the lock including at least one holding element that interacts with the dosing mechanism, or with a dosing element of the dosing mechanism, whereby an adjustment movement of the dosing mechanism or the dosing element is prevented in a starting position of the lock and is possible only after a movement or displacement of the lock or the holding element. An injection device used in conjunction with a two-chamber ampoule is encompassed, as is a method for preparing the injection device for dispensing a substance wherein the ampoule is introduced into the injection device and a lock is released when the ampoule has been introduced far enough to appropriately mix the substances in the two chambers, whereupon the mixed substances can be dispensed from the ampoule.